MEXICO CITY -- Mexico's oil has long been a source of national pride. Now, with reserves dwindling, President Felipe Calderon has floated an initiative to rescue the government oil giant, Pemex: allow foreigners to help the company drill for oil.

Pemex says it needs outside technological help to bring up oil from deep-water fields. But such action raises the question of whether foreign companies should be allowed to invest and profit from any of Mexico's oil riches.

The issue is emotionally as well as politically charged. Mexico nationalized its oil fields in 1938, expropriating American and European companies. Ever since, most Mexicans have considered public ownership of the country's most lucrative natural resource to be a cornerstone of their sovereignty.

On Tuesday, leaders of the leftist Democratic Revolution Party (PRD) announced that they would form "resistance brigades" composed of 10,000 women and 18,000 men to fight Calderon's proposals with marches and barricades.

And Wednesday, leaders of the Institutional Revolutionary Party, or PRI, expressed opposition to key elements of the reform after suggesting earlier that they might support it. Calderon needs PRI votes to pass any legislation.

"We are in complete disagreement with the (Calderon) government," Manlio Fabio Beltrones, a PRI senator, said in a radio interview.

Analysts say Mexico's oil reserves could disappear in a decade if no new fields are tapped. The country's main oil field, Cantarell, is in the throes of a major decline. Located in the shallow Gulf waters off Campeche state, Cantarell's output in February averaged 1.145 million barrels per day, according to government figures. That's off 24 percent compared with the same month last year. And it's down 46 percent since production peaked at about 2.1 million per day in 2004.

In an interview with the Los Angeles Times in February, Calderon suggested that Mexicans should not look upon foreign investment in the oil industry with fear. He pointed to the examples of China and Brazil, countries with government-controlled oil companies that allow foreign investment.

"We have a treasure that could finance the development of the country," Calderon said. "The problem is that it's in the ocean and at a depth that we Mexicans cannot yet reach by ourselves."

The "treasure" metaphor has been repeated in Pemex television commercials airing this week in support of deep-water drilling. Those ads say Pemex can reach deep-water oil with "the experience and technology of others" but does not specifically refer to foreign companies.

The PRD has countered with spots that call any private profit from Pemex a national betrayal.

Andres Manuel Lopez Obrador, the leader of the PRD, told thousands of people at a protest rally Tuesday here that they should resist "the group of potentates, both foreign and domestic, who cynically ... plan to transform our oil into a juicy private business."

Last year, Pemex generated $104.5 billion in revenues. Half of that money went to the government: The company is the nation's largest taxpayer, and Mexico relies on it to fund schools, roads and social programs.

After paying salaries and other costs, Pemex lost $1.5 billion -- in a year when major oil companies posted blockbuster earnings.

PAN officials have not submitted their proposed legislation. But they have given the broad outlines of reforms to allow private companies to sign contracts with Pemex for exploratory drilling.

PRI officials, who appeared initially warm to the idea, said this week they could not support reforms that guarantee investors a share of any oil they find.

Such arrangements are standard in the oil industry, embraced even by Cuba. But they are impossible under current Mexican law, which says the country's oil deposits belong exclusively to Mexico's citizens. The nation's constitution allows private companies to participate solely as contractors and forbids so-called "risk contracts" that would reward them with a share of production.

Analysts say private investment is unlikely unless Mexico agrees to share profits from drilling and exploration, known in the industry as "upstream" operations.

"Any upstream component of an energy reform has been difficult from the beginning," said Enrique Bravo, Latin America analyst with the Eurasia Group, a New York-based political risk analysis consulting company. "Now it looks even more so. ... The PRI is toughening its position."

Indeed, Beltrones, the PRI's leader in the Senate, accused the government of "alarmism" Wednesday. In an interview with the newspaper Reforma, he said Calderon officials are painting the worst possible picture of Pemex to ram through its energy reform.

Proven reserves have fallen 41 percent since 2000, to about 14.7 billion barrels, according to Pemex figures. That translates into less than a decade's worth of production unless Mexico can tap new sources of oil.

Independent oil analysts say the country's petroleum bounty is rapidly evaporating, having predicted the decline of the Cantarell field. The real surprise, they say, is how little Mexico has done to prepare for Cantarell's inevitable demise.

Geologists believe there might be reserves of oil to be found miles below the surface in the deep waters of the Gulf. Deep-water drilling requires specialized know-how, advanced technology and loads of money -- none of which Pemex has, analysts say.