The proposal, which the president hopes will yield a long-term economic payoff, would change the Mexican Constitution so that private companies could partner with the state monopoly, Petroleos Mexicanos, or Pemex, and share in its profits. The plan instantly came under attack, both from leftists who fear it threatens the nation’s patrimony and from those on the right who believe it doesn’t go nearly far enough.

Pemex was created in 1938 to manage all Mexican oil and gas resources after then-President Lazaro Cardenas expropriated and nationalized foreign petroleum holdings in the country.

The move made a hero of Cardenas, but the company became notorious for corruption, inefficiency and bloat. In the last decade, it saw its production steadily diminish as the country began tapping out its once-vast stores of easy-to-find oil, much of which was beneath shallow waters in the Gulf of Mexico.

Many analysts say the country needs foreign investment and expertise to help it extract oil and gas from more technically challenging deepwater wells and shale rock formations. But the idea is passionately opposed by many who fear the country is on the road to privatizing its most important natural resource. In a recent national poll, 65% said they opposed foreign investment in Mexico’s petroleum industry.

In a pomp-filled presentation Monday before a handpicked audience at Los Pinos, the Mexican White House, Peña Nieto labored to assuage those fears. His reform, he said, would create jobs, lower electricity and gas bills, and even result in better food, because of the development of new fertilizers. And he insisted that Mexico would retain “absolute control” over its oil.

“This is the moment to utilize all of our energy to move and transform Mexico,” he said. “Petroleum and other hydrocarbons will continue to be the exclusive patrimony of the nation. We will continue being owners of the petroleum income.”

Peña Nieto’s allies say the stakes could not be higher for the Mexican economy. Pemex is responsible for about one-third of the government’s income. But if current trends are not reversed, a country famous for its abundant oil and gas reserves could become a net energy importer by 2020.

Energy Minister Pedro Joaquin Coldwell said Mexico had to change to meet growing demands both abroad, in places like China, and domestically.

But the political stakes are daunting as well. Energy reform is perhaps the most important element of a broader package that Peña Nieto is hoping will transform his country, making it more modern and efficient.

With the support of his own Institutional Revolutionary Party, or PRI, and members of the conservative National Action Party, the proposal is expected to pass both houses of Congress. After that, 17 of Mexico’s 32 state and Federal District legislatures must approve the amendment.

Even if he is successful, Peña Nieto, who took office in December and is limited to one six-year term, could still suffer politically if the opposition mounts a sustained and impassioned protest. It is difficult to say whether Mexico’s left, which is suffering from internal divisions, can bring such heat. But there is a concern that, with emotions rising, things could get ugly.

“The debate over energy reform represents the largest challenge in the history of the young Mexican democracy,” columnist Juan E. Pardinas wrote in the newspaper Reforma on Sunday, referring to the period since 2000, when the PRI was defeated after decades of one-party rule. “The civility of the political actors and the power of our institutions will be under a difficult stress test.”

The opposition has been gearing up for what it hopes will be an ideological battle to be waged, at least partially, in the streets. Leftist leader Andres Manuel Lopez Obrador, who was able to essentially shut down the Mexican capital with protests for weeks after losing the presidential race in 2006, has called for a rally next month, about the time that Congress will be debating the legislation.

“What is it they are trying to do with the privatization of Pemex?” Lopez Obrador said in a video statement released Monday. “They are trying to make off with the wealth of the Mexican people.”

Jesus Zambrano, head of the leftist Democratic Revolution Party, criticized Peña Nieto’s proposal to change two articles of the constitution. If the government really does not want to privatize Pemex, he said, there should be no need to fiddle with the document. Earlier this summer, Zambrano’s party held a rally under a giant statue of the late President Cardenas, arguing that opening the company to foreigners would betray his great achievement.

But Peña Nieto argued Monday that his proposal was in the spirit of Cardenas’ reformist impulse, noting that the former president had originally intended Pemex to be able to work with foreign companies. Such partnerships were only prohibited, Peña Nieto noted, by a constitutional amendment in the 1960s.

His presentation was in many ways attuned to notions of national pride. It opened with the president and his Cabinet delivering a full-throated version of the national anthem. Cabinet members, including smartly dressed military officers, were introduced by name, and stood while being applauded.

On the right, meanwhile, critics say the president’s proposal doesn’t go far enough.

“Very timid,” said Gustavo Madero, head of the conservative National Action Party, or PAN, who argued that the measure fails to end the monopoly that Pemex enjoys, and fails to open the company to meaningful competition.

The PAN had proposed a plan that would go beyond the PRI’s idea of sharing profits with private companies. Its proposal would allow for the granting of concessions — that is, contracts in which the government, for a fee, grants a company the right to explore and produce oil in a given area over a fixed period. Private companies often prefer concessions over profit-sharing arrangements.

Rafael Caro Quintero was a high-rolling drug lord sent to prison for the 1985 kidnapping, torture and slaying of an agent with the U.S. Drug Enforcement Administration.

Raul Salinas, the brother of a former Mexican president, was a free-spending playboy. He was convicted, and then acquitted, of the 1994 killing of a top politician. He was also suspected of stealing millions of dollars from the government.

But in recent days, the old criminal cases against these two characters have fallen apart, leaving many dismayed by what they perceive as the enduring dysfunction of the Mexican justice system.

Even though Mexican officials, with U.S. advisors and millions of dollars of U.S. aid, have focused in recent years on reforming the system, these two cases are likely to bolster the suspicion that the law still does not apply to the well-connected and the powerful — either because of their enduring influence, or the incompetence of those who pursue them.

Historian and academic Lorenzo Meyer in a magazine interview this week said the Salinas ruling last month was, in effect, a message to the Mexican people:

“Listen, you drooling pack of idiots, the true political situation is this … at the end of the day, we, the political class, understand the nature of corruption in this country, and it’s not going to change.”

Though spectacular meltdowns of major criminal cases are common in Mexico, these were standouts, with each suspect, in his way, shaping the course of history.

Salinas’ Champagne lifestyle — his numerous luxury properties, and millions stashed in overseas bank accounts — may have contributed to the public’s rejection of the Institutional Revolutionary Party, or PRI, the once all-powerful institution that counted the Salinas family as influential members.

Caro Quintero was a legendary drug lord. A Times article from 1992 mentions how a witness in a criminal trial described him at a lavish party smoking cocaine on the back of a prancing horse.

On Friday, Caro Quintero, a founder of the Guadalajara drug cartel, was freed on a technicality after 28 years behind bars. A panel of judges declared that the slain DEA agent, Enrique “Kiki” Camarena, could not be considered a member of the diplomatic corps; as a result, the judges ruled, the case should not have been tried in federal court.

In Washington, the DEA said it was “deeply troubled” by Caro Quintero’s release and indicated the U.S. government would attempt to have him extradited to face charges in the United States.

“Caro Quintero was the mastermind and organizer of this atrocious act,” the DEA said in a statement, referring to the torture and slaying of Camarena. “We are reminded every day of the ultimate sacrifice paid by Special Agent Camarena.”

Although other drug kingpins and cartels have eclipsed the 61-year-old Caro Quintero, U.S. officials believe he has continued to run trafficking and money-laundering operations from prison. In June, the Treasury Department added several of his alleged associates to its so-called kingpin list, imposing sanctions on 18 people and 15 companies.

The Treasury Department alleged that Caro Quintero’s money was bankrolling, among other things, real estate projects, a luxury spa, a shoe store and a swimming pool company.

The cases against Caro Quintero and Salinas collapsed less than a year into the government of President Enrique Peña Nieto, whose victory last year marked the return of the PRI after a 12-year absence.

The party ran the country as a quasi-dictatorship for much of the 20th century, a time when the courts were often used as a tool for the party’s political agenda. Peña Nieto promises that the party has become more transparent and democratic. But he will have to work particularly hard to prove it, given the sins of his party’s past old-school leaders, known as the “dinosaurs.”

Further complicating the situation is the fact that Peña Nieto is part of a political clique closely linked to the powerful Salinas family. Peña Nieto’s attorney general plans to appeal the ruling in the Salinas case. But that hasn’t stopped some from speculating that the fix is in.

“Damn if we aren’t returning to the times of Jurassic Park,” Twitter user Robert Cabrera Vera wrote Friday. “…a few days ago, Raul Salinas and now Caro Quintero.”

Salinas, whose brother Carlos Salinas was president of Mexico from 1988 to 1994, was exonerated last month of the charge of “illicit enrichment” by federal judge Carlos Lopez Cruz. Known as el hermano incomodo, “the inconvenient brother,” Raul Salinas dominated Mexican headlines for years for living like Mexican pasha, even though he was employed as a government bureaucrat.

He was also found guilty of the 1994 slaying of Jose Francisco Ruiz Massieu, a PRI leader. Salinas spent 10 years in prison for the crime, but was acquitted and freed in 2005.

The seizure of foreign oil companies 75 years ago that created the company is a cause for annual celebrations affirming Mexico’s fierce sense of independence from outside interference.

Yet even as the country’s new president, Enrique Peña Nieto, credits Pemex with building the nation, his administration acknowledges that the notoriously inefficient conglomerate is in trouble: If it is not opened to private and foreign investment, Mexico, the world’s ninth-largest oil producer, will become a net energy importer by 2020, officials say.

As Peña Nieto moves ahead with a plan to overhaul Pemex, he is navigating the most perilous political minefield of his young presidency. He is toying with taboos and challenging revered perceptions surrounding the nation’s top revenue earner. And he is meeting with impassioned opposition.

At the back of a recent rally for Pemex, Jesus Castillo Sanchez, a 46-year-old handyman, waved a giant Mexican flag as if he’d just taken a hill in battle. Booting the foreign oil companies in 1938 “gave Mexico its true independence from the great powers,” Castillo said. “After [the foreigners] bring their oil platforms, they will bring their armies and their troops.”

The president is expected to introduce landmark energy reform legislation, including proposals addressing Pemex, as early as this week.

The government and industry experts contend that Mexico needs advanced technical expertise from outside companies to find and retrieve oil and gas from deep water and shale-rock formations that are believed to hold more than half the country’s estimated 14 billion barrels of reserves.

But “Pemex is not allowed … to choose associations … to reduce the level of risk that you run” in deep-water exploration, Carlos Morales Gil, Pemex director of exploration and production, said in an interview. “What Pemex needs is budget autonomy and flexibility” to form joint ventures, he said.

Making that possible could require changing the constitution, and that could prove a bruising battle for Peña Nieto. The matter is so sensitive, so wrapped up in Mexico’s ability to assert its independence from foreign meddling, that when Peña Nieto, speaking in London, suggested that the other major political parties had already agreed to reform, several politicians back in Mexico went ballistic.

Yet the problems plaguing Pemex are legendary. With corruption, poor management, a union that demands enormous benefits and a corporate structure that fosters duplicate jobs, Pemex is a model of how not to run an oil company.

According to a 2011 study by the Texas-based Baker Institute, Pemex, with about 140,000 employees, produced revenue of $585,000 per employee, a quarter of the per-employee revenue of British oil giant BP and about half that of the partially state-owned Brazilian oil company Petrobras.

Morales, the Pemex executive, acknowledged that Mexico was 30 years behind the industry standard in deep-water exploration. Pemex, he said, hadn’t felt pressure to pursue the riskier search because it had so much readily accessible shallow-water and inland oil. Consequently, Mexican engineers did not keep up with the widening technological know-how that has benefited competitors.

A case in point is the Cantarell oil field, in the Bay of Campeche. It was discovered in 1976 by a fisherman who noticed his nets were being ruined by oil. Pemex engineers soon realized he had stumbled on one of the largest oil fields in the world.

But in the mid-2000s, production at Cantarell peaked, and it has declined rapidly, while a second big find, Ku-Maloob-Zaap, has already been bled of two-thirds of its reserves.

In all, Pemex’s production of crude has declined steadily to 2.6 million barrels a day — from a high in 2004 of 3.5 million — stabilizing only in the last year or so.

There’s plenty more out there, but experts say Mexico can’t get to it without outside help.

“Mexico, outside of the U.S., probably has the widest array of energy plays that anybody has in the world: shallow water, deep water, shale gas, shale oil, tremendous marginal fields that have largely been neglected over the last 40 years,” said John Padilla, managing director of the international energy consulting firm IPD Latin America. “It’s too much for any one company to handle. There’s nobody in the world that could do that.”

The Mexican Constitution says that raw materials found in the land, like oil and gas, belong to the state, requiring a constitutional amendment for future agreements that might touch on ownership of resources.

Private operators could pay a tax, based on percentage of revenue or profit, for the right to pump. Or they could enter into joint-venture agreements with Pemex, each side sharing risks and benefits.

But how outside investors share revenue, and Mexico’s long-standing refusal to cede any ownership of the oil itself, remain a key obstacle.

The problem is that companies probably would prefer to take a cut of the oil itself; the amount of “booked reserves” to which a company has the legal rights is used by analysts to set the company’s stock price.

The proof lies in this dusty border town of 14,000 people. Here, unmonitored goods and travelers float across the wide Suchiate River — the boundary between Guatemala and the Mexican state of Chiapas — on a flotilla of inner-tube rafts. They cross all day long, in plain sight of Mexican authorities stationed a few yards upriver at an official border crossing.

Some of the Central Americans are visiting just for the day. Others are hoping to find work on Mexican coffee plantations or banana farms. But many will continue north toward the United States.

Photos: Anarchy along Mexico’s southern border crossings

There is no guarantee they will ever get there. Lying in wait are Mexican criminals, and even Mexican officials, who aim to kidnap northbound travelers, extort money from them and sometimes even rape and kill them. About 10,000 such migrants have disappeared in Mexico every year since 2008, according to Mexican government estimates.

Luis Martinez, a 33-year-old migrant from Honduras, had crossed the Suchiate without incident early one morning in June. He knew he was risking his life by moving on. But he said that the crippled Honduran economy left him little choice. He needed to work. He needed to earn.

“It’s going to be hard,” he said. “We’ve heard about it all: the cartels, the kidnappings, the robberies, everything.”

Along with drug violence and the slaying of journalists, the ugly fate of the Central American migrant has become one of the darkest stains on Mexico’s reputation.

In recent weeks, however, the government of President Enrique Peña Nieto has pledged to bring new order to the border region. Interior Minister Miguel Angel Osorio Chong said the new plans will “ensure that [migrants] don’t suffer.”

Security at border crossings will be strengthened, officials say, and sophisticated “internal control” stations will be set up a few miles inside the country along commonly-used migration routes, to better monitor the movement of goods and people.

The government is also expanding a program to record the fingerprints, photos and other identifying features of migrants, the newspaper Milenio reported. High-tech biometric kiosks, partially paid for by the U.S. government, will record the data of those applying for temporary visitor and work permits.

Meanwhile, the Mexican navy, which is charged with protecting the border, has deepened its partnership with the Guatemalan military, staging coordinated operations in an effort to run off the drug cartels that have taken advantage of the border’s numerous unpoliced crossing points, Vice Adm. Francisco Ramon Tiburcio Camacho said.

The outlines of the plan raise many questions. The biometric kiosks, for example, may help authorities keep track of migrants who have been caught by authorities, or who willingly apply for legal entry. (Among other things, the data can help identify their bodies if they are found in unmarked graves). But how will they help keep tabs on the many migrants who are evading Mexican authorities?

More generally, it is unclear whether the new plans will really allow Mexico to get a handle on a border that is more than 700 miles long, and which experts say is essentially untamable. Border fences and walls, they say, would be almost impossible to build through the deep jungle and stark mountainous terrain traversed by the line separating Mexico from Guatemala and Belize.

The Mexicans reject the idea of building walls anyway, having long maintained that the U.S. southern border strategy, with its imposing barriers and armed agents, amounts to conduct unbecoming a neighbor. In a speech June 3 near the border, Osorio Chong promised that no walls would be built under the new plan. It would be focused on human rights, he said, and more in keeping with “what Mexico demands north of its border.”

Sunday’s midday shooting death of Vice Adm. Carlos Miguel Salazar Ramonet and one of his assistants has sent shock waves through Mexico. The death of such a high-level military officer is a rarity in Mexico’s drug war, and the navy is considered the nation’s most elite and trustworthy fighting force.

The attack was one of numerous violent assaults on federal authorities in the state of Michoacan in recent weeks. Seven federal police officers have been killed, and many more wounded. More than 20 alleged criminals have also died in the shootouts.

Many here assume that the attacks were coordinated and carried out by the Caballeros Templarios, or Knights Templar, a cartel that runs an extensive and violent extortion racket in Michoacan. The Knights may be lashing out in response to the increased number of federal forces that began arriving in May to try to regain territory that the cartel essentially controls.

It appears that Salazar was not in Michoacan on assignment. At a news conference, Mexican Atty. Gen. Jesus Murillo Karam said that Salazar had been visiting family members in an undisclosed part of Mexico and was being driven back to Puerto Vallarta, in the neighboring state of Jalisco, where he had been based. He was traveling in a white Chevrolet Suburban with his wife, his assistant and a driver.

When Salazar discovered that the freeway he was traveling on was blocked, he decided to take a secondary road, where another truck blocked his path, Murillo Karam said. The people in the truck began asking the Suburban’s occupants for identification, when a second truck pulled up. The people inside the second truck opened fire on the Suburban.

The vice admiral covered his wife, getting her to the floor of the vehicle. But he was killed, along with the assistant, while trying to fight back, Murillo Karam said.

The wife and the driver were injured, but were “out of danger” as of Monday afternoon, according to Eduardo Sanchez, a government spokesman.

The three men arrested for taking part in the attack confessed their involvement, Murillo Karam said, “or at least [to being] the occupants of one of the trucks.” The men, according to the prosecutor, also acknowledged working as robbers, kidnappers and extortionists for the Knights Templar, earning monthly salaries of 7,500 pesos, or about $590.

That detail that goes a long way toward explaining the allure of the cartels in a country where minimum wage workers  earn roughly 1,430 pesos per month, or $112.

Murillo Karam said it was “important to note that the admiral wasn’t wearing a uniform, that he was going like any other citizen, much like many citizens who have visited their family on the weekend and were coming back to go to work.”  But the attackers might have deduced that Salazar was a top government official, particularly given his use of the Suburban, a favorite car among Mexico’s elite.

Whether Salazar was singled out or picked at random, the episode highlights the challenge President Enrique Peña Nieto faces as he tries to bring peace to Michoacan, a state he has acknowledged is partly controlled by cartels.

On Monday, Peña Nieto said the attack and incidents like it reaffirm the need to impose the rule of law throughout the country.

Avila, 52, was arrested in 2007 in Mexico City with her Colombian boyfriend, Juan Diego Espinoza Ramirez, who pleaded guilty to cocaine trafficking charges two years later. A Mexican judge acquitted Avila of drug-related charges in 2010, but she was extradited to the United States to face charges that she conspired to import more than 11 pounds of cocaine into the U.S.

Officials alleged Avila was an important link between Colombian cocaine suppliers and Mexican drug groups. It is rare for a woman to face such charges, and this fact, in addition to her glamour, vanity and smiling insouciance — she once had a doctor visit her in prison to give her Botox treatments — turned her into a kind of drug war folk figure. She was interviewed by CNN’s Anderson Cooper, and the Mexican musical group Los Tucanes de Tijuana wrote a well-known narcocorrido about her.

Avila has always claimed she was an innocent homemaker. In a plea agreement with prosecutors in April, the drug charges were dropped, and Avila admitted to an accessory charge for helping to hide her boyfriend from authorities, according to Avila’s U.S. attorney, Stephen J. Ralls.

Ralls, in a phone interview Thursday, said that U.S. District Judge K. Michael Moore, in sentencing Avila, also gave her credit for time served while in custody in Mexico and the U.S. She was still behind bars Thursday evening, but Ralls said it is probably just a matter of days before she is freed.

Ralls said that U.S. officials would probably send her back to Mexico soon, where she is not facing any further charges. Alberto Islas, a spokesman for the Mexican attorney general, said that his office “has no order of apprehension against Sandra Avila, nor is there any inquest open against her.”

Authorities first became suspicious of Avila in 2001 after her son was kidnapped and she was reportedly able to pay a multimillion-dollar ransom to get him back.

Cecilia Sanchez of The Times’ Mexico City bureau contributed to this report.