The recent plunge in oil prices has jeopardized Mexico's shaky economic recovery and debt-payment plans, reviving the risk that this major Latin American nation could renege on loan commitments unless it receives fresh financial help, according to western bankers and energy analysts.

The world's fourth-largest oil producer, Mexico, with a population of 76 million, has been hard hit by dwindling revenues and soaring debt -- and it has few other sources of foreign exchange.

The latest drop in oil prices to below $20 a barrel threatens to erode benefits from the country's austerity regimen, intensifying political and economic discontent that has been cited as a factor in illegal immigration across the U.S. border.

No other large nation outside the Organization of Petroleum Exporting Countries is as dependent upon oil as Mexico, economists here note. Since 1980, oil consistently has provided at least two-thirds of the country's total export earnings and half of the federal government's revenues. Every dollar-per-barrel reduction in the price of oil represents a $550 million drop in Mexico's anticipated yearly hard currency income.

"The market is crazy now, but if the numbers stay that low, Mexico is going to have to make a major, major price change this month," said a U.S. oil company official who has negotiated purchasing contracts here since Mexico began exporting on a large scale in 1977. "This is the most difficult time for the Mexican oil industry that I have ever seen."

Mexico's last posted prices of $26.25 for light crude and $22 for heavy oil are both about $5 above current spot market rates, foreign oil company buyers here said today. Because of the market's volatility, Mexico has postponed the announcement of its oil prices for January until the end of the month, when they will be set retroactively.

With a foreign debt of $96 billion, much of it accrued by the Pemex state oil monopoly as it developed the fields that are now pumping crude, Mexico has sought to avoid further borrowing.

If Mexico's oil prices do fall by as much as $5, a U.S. bank negotiator said in a telephone interview today, the country will not be able to meet its projected commitment of more than $14 billion in debt interest and principal payments this year -- a figure equivalent to Mexico's officially but perhaps optimistically estimated 1986 petroleum income.

"If prices drop that far, they are going to have to sit down and redo all their calculations" for foreign debt servicing and domestic spending, said the banker. Mexico's 1986 budget, released in November, assumed an average $2.12 price drop below the average then, $24.50 per barrel.

Mexico's December prices were chopped retroactively since then by an average $.90 per barrel, and customers say they are demanding a much deeper reduction this month.

"That $2 cushion is already wiped out," a Western European oil buyer said in an interview here this week. A New York banker, citing a recent in-house energy market survey, said Mexico's creditors "cannot rule out as unthinkable" the possibility of an average $15 price level for Mexican oil by 1987.

Energy Minister Francisco Labastida Ochoa, calling Mexico a "victim" of OPEC overproduction and discount pricing, declared today that the country nonethless could pump oil at much lower prices due to its low production costs -- said by ministry officials to be less than $5 per barrel.

But the oil price collapse "is already affecting us" economically, Labastida added, and could potentially provoke "chaos in international financial markets" if it prevents Mexico and other oil exporters from repaying foreign loans.

Sen. Jose Patrocinio Gonzalez Blanco, suggesting that falling oil prices might force Mexico to suspend interest payments, said yesterday this "could be a reason to renegotiate the debt" now on more favorable terms.

Last Aug. 29, in what was said then to be the largest single transaction in modern banking history, Mexico signed an agreement rescheduling the payment of fully half of its debt over 14 years.

Mexican officials are now scheduled to meet formally in New York Feb. 4 with the steering committee representing the country's biggest private creditors to discuss their request for $2.5 billion in new commercial loans this year. Mexican officials had hoped to receive the first disbursement from the new credit by February or March, but steering committee members now say Mexico is unlikely to receive any fresh money before June.

The committee member said there is consensus among bankers that Mexico will again miss by wide margins all its announced fiscal and macroeconomic targets.

Before granting new credits, he said, bankers are demanding substantive "structural reforms" that go beyond the budget-cutting demands of the International Monetary Fund -- with which Mexico is expected to sign a new standby loan accord in February.

"We're convinced that we have a sick patient here, and if we don't do something to cure the disease we're going to catch it ourselves," the banker said. The biggest change bankers say they want to see is a reversal of capital flight, estimated to have drained $20 billion in oil and foreign loan money out of Mexico since 1980.

But the investment climate needed to lure back that money will be difficult to achieve as falling petroleum prices erode Mexico's foreign reserves and depress industrial output, bankers and economists acknowledge.

With falling oil prices and no fresh private credit, Mexico in the first half of 1986 "will find it extremely difficult" to dent the deficit, which is "by far the biggest macroeconomic problem Mexico now faces," a foreign diplomatic analyst here said.

In 1985, Mexico announced its intention to lower the deficit to 5 percent of gross domestic product but finished with a deficit twice that size. President Miguel de la Madrid has pledged this year to halve the deficit, again to 5 percent, but many analysts predict the deficit will remain at about 10 percent of GDP.

There is a widespread "attitude of distrust and skepticism" about the government's commitment to its proclaimed economic goals, said Claudio Gonzalez, president of the Business Coordinating Council, Mexico's largest business-sector association, in a speech this month.

Many economists here say they are more concerned about soaring peso interest rates and politicized monetary emission policies than they are about the world petroleum market. But the single most critical variable for the Mexican economy "is still the price of oil, and it is completely beyond Mexico's control," a European diplomat remarked.

Exacerbating the economic hardship of the recent price slide has been Mexico's "inflexible and bureaucratic" monthly oil pricing system, a Western European oil company official charged, voicing a common industry view.

By delaying inevitable price cuts, Mexico in 1985 lost about $1 billion in contracts, bringing export volume down to an average 1.44 million barrels daily, as compared to the 1.53 million barrels per day it averaged in 1983 and 1984. Without such a pricing reform, he predicted, it will be "very difficult" for Mexico to achieve its goal of an average 1.5 million barrels daily in 1986.

A related concern raised by oil industry veterans here is Mexico's declining investment in exploration and new well development -- a matter of immediate strategic interest to the United States, which has relied for most of this decade on Mexico as its biggest foreign oil supplier.

Explosive growth of Pemex in the late 1970s was financed by direct foreign loans, which are no longer available. With most of Pemex's oil income now devoted to the servicing of its own and other government debts rather than to drilling, the monopoly's production capacity has declined to perhaps 2.8 million barrels per day from its 1982 peak of about 3 million, employes and foreign experts estimate.

A Mexican congressional committee recently warned that at its present pace of consumption and development, Mexico could be forced to start importing oil in 15 years, despite its present 50 billion barrels of proven crude reserves.

These charges have been publicly echoed by leaders of Pemex's union, the wealthiest, most powerful and reputedly most corrupt of the many labor bodies affiliated with the ruling party.

In an unusual personal confrontation with de la Madrid, union Secretary General Jose Sosa charged two weeks ago during a ceremony at the presidential residence that Pemex "is spending more money on paperwork and high management salaries" than it is on maintenance and production. "If Pemex goes under," said Sosa, directly addressing de la Madrid, "so do you, so do we all, and so does the country."