MIAMI, DEC. 31 -- Cuba, already seized by its most severe economic crisis in a generation, faces trials in the coming months that could pose the gravest threat in 30 years to the survival of President Fidel Castro's socialist revolutionary rule.

While wary of predicting the imminent demise of a politician of Castro's skills, analysts here and in Latin America say Castro is not offering a coherent program to deal with the free fall of the Cuban economy -- which is thought to have shrunk by about 5 percent in 1990, largely because of upheaval in the Soviet Union.

These U.S. and Latin American specialists say prospects for 1991 are more troubled, making it the most critical year for Castro's revolution since 1961. Castro came to power on Jan. 1, 1959, after which relations with the United States soon deteriorated.

In 1961, Cubans were stunned by a break in relations with Washington and a U.S.-sponsored armed invasion at the Bay of Pigs -- all before the Soviet Union had made a firm decision to underwrite Castro's fledgling revolution. The following year, the world faced a prospect of a nuclear war before Washington faced down Moscow in the Cuban missile crisis.

"No year since then has been as difficult and critical as 1991 will be," said Wayne Smith, director of Cuban Studies at the Johns Hopkins University School for Advanced International Studies in Washington and former head of the U.S. Interests Section in Havana. "Cuba was at a crossroads in 1961-62 and now, 30 years later, it will be again."

"What makes the coming year unprecedented are the problems in the Soviet Union," said Jorge Dominguez, a Cuba scholar at Harvard University. "One needs to go back to 1968, when the Soviets slowed down delivery of petroleum products because of variety of disagreements. . . . That was tough, but there was still a Soviet Union by the end of the year. Now it's not clear the Soviet Union would have the capacity to help Cuba even if it wanted to."

In September, Castro, noting the threat of the Persian Gulf crisis and soaring oil prices, said: "It is impossible to foresee what the situation will be in 1991. . . . We might have to work with less and less and even with zero."

The grim outlook reflects a confluence of several major factors:

The deepening economic crisis and pressures for economic change in the Soviet Union, Cuba's main benefactor.

Moscow's decision to shift all trade with Cuba and its other trading partners from a subsidized barter to a hard-currency basis, beginning Wednesday. Depending on the prices set for Soviet oil and Cuban sugar, the shift could mean a staggering new blow to the Cuban economy, whose hard-currency reserves are already critically low.

The fourth Cuban Communist Party Congress, originally scheduled for the first half of the year. At the congress, Castro is expected to showcase a number of changes that are already generating high hopes for a recovery. But few expect them to go deep enough to transform the centralized economy.An expected influx of visitors and resultant international scrutiny in August, when Havana hosts the Pan American Games.

The economy already is buffeted by dwindling imports from Eastern Europe and the Soviet Union. There are shortages of food, energy, paper, transportation, household goods and tires.

A 20 percent cut in Soviet fuel shipments this year has had a direct effect on life in the island nation of 10.4 million people. Horse-drawn carts are replacing delivery trucks in Havana and oxen are replacing tractors on the farms. Cuba has ordered 700,000 bicycles, mostly from China, to wean motorists from their cars.

Analysts contend that only a radical restructuring to introduce free markets, material incentives and private ownership could rescue the economy in the long term.

But Castro, 64, has given no indication that he is willing to abandon three decades of anti-capitalist rhetoric and policy to embrace perestroika, or restructuring. If he was ever tempted to do so, a glance at President Mikhail Gorbachev's troubles in the Soviet Union might suggest that such changes can loose forces that ultimately may be uncontrollable.

"The regime has seen what happens with a decentralizing reform in Eastern Europe," said Andrew Zimbalist, an economist at Smith College. "It would be a bloody mess economically and politically, just 90 miles from the United States. If they tried to do it, it would be extremely destabilizing."

Even as communist regimes have crumbled around the world, Castro has vowed to preserve Cuba's system, defending it as a more humane and equitable arrangement for society.

"If they told me that 98 percent of the people did not believe in the revolution, I would carry on fighting," Castro told the Union of Young Communists in an Oct. 27 speech. "Because a revolutionary must be a man who, even if he is left alone, continues to fight for his ideas."

His perorations usually are punctuated with the apocalyptic slogan, "Socialism or death!" Lately, the words have been accompanied by action. In 1990, Castro proclaimed a "special period in time of peace" -- a draconian austerity program designed to put the economy on a wartime footing. Cuban officials have said the special period might last five years, and Castro has warned of "very difficult and very harsh trials" ahead.

The program is designed to cut oil consumption and increase food production while preserving the few industries in Cuba, such as tourism and biotechnology, that generate revenue.

Factories, including one of the island's three nickel plants, have been shut to save fuel. Sales of refrigerators and air conditioners have been suspended. Authorities have suggested that housewives do their cooking with wood or charcoal. And all around Havana, street lights are dimmed in the evening.

In Havana, a city of 2 million, the number of taxis was halved to 2,000, with three-quarters of those assigned to ferrying passengers to and from hospitals. Bus and train schedules have been slashed. Aeroflot, the Soviet airline, is shifting its Latin American hub to Miami because jet fuel is in such short supply in Havana.

Some cuts result in a sharp drop in production. "If we must reduce the work week, we will reduce it," Castro said in a Sept. 28 speech in Havana's Karl Marx Theater. "If we cannot work five days, we will work four. And if we cannot work four, we will work three and give time off."

Already, delivery problems from Eastern Europe and the Soviet Union have forced authorities to tighten distribution of food, clothing and household appliances and expand the number of rationed items.

Goods added to the ration list in September include canned meat and fish, crackers, soy sauce, ketchup, ham, bacon and sausage. Many cannot be found on store shelves even though their distribution is now officially controlled.

To boost food production, thousands of bureaucrats are being moved from office jobs in Havana to farm jobs in the countryside, the Miami Herald has reported. Moreover, Castro has announced that some 20,000 Havana residents will work rotating stints outside the capital to boost agricultural production. Many taxi drivers whose jobs were eliminated to save fuel now are to be farmers.

Castro has managed austerity in the past, as in the late 1960s when he placed the civilian economy under military command. His personal stature and political passion remain formidable, and many Cubans give him a nod of respect even while grumbling about the failures of the system he constructed.

Moreover, his regime's control over virtually every aspect of life remains unchallenged. Dissident groups are not permitted to grow into a threat. Block committees continue to function as the Communist Party's eyes and ears in every neighborhood.

It remains difficult to imagine what spark could ignite a popular protest against Castro in such a tightly controlled society. But in the history of the revolution, Castro has never faced such a combination of economic hardship, fading hopes for the future and shrinking Soviet aid.

"It's very hard to motivate people with the idea of a better future, as they did in the past," said Harvard's Dominguez. "That message doesn't make any sense to people now. It's that grimness that puts the regime at risk."

There are signs that Castro, despite his anti-capitalist rhetoric, is taking tentative steps toward making competition a factor in the economy.

Some high-ranking officials close to Castro, such as Carlos Aldana, Communist Party ideology chief, and Eugenio Balari, chief of internal demand, have hinted that the leadership is considering turning some services and small-scale manufacturing enterprises over to private owners. To compensate for the resulting inequities of wealth, the regime is weighing a limited system of progressive taxation.

In his Sept. 28 speech, Castro hinted at incentives for workers "who do more, and a little less in the rationing card to the lazy."

There has already been some foreign private investment in hotels, which Castro values as a source of dollars. Workers in the new hotels may also be paid a premium as an incentive to deliver better service.

In the last year, there have also been some limited steps toward decentralization. According to Smith College's Zimbalist, the number of manufactured goods coming under central planning was reduced by two-thirds to 100, and state enterprises were cleared to exchange some 600 products without going through the bureaucracy.

But the sum of the changes, analysts say, is trivial given the scope of Cuba's economic troubles. "The Cubans are going to try reform, but very piecemeal, step by step and very controlled from above," said Zimbalist.

None of this is likely to make up for the dwindling assistance from Moscow, variously estimated between $1.5 billion and $4.5 billion annually.

When the Kremlin decided more than a year ago that it would end its fuel subsidy to Cuba and Eastern Europe on Jan. 1, 1991, the Soviets indicated they would try to soften the blow to Cuba by negotiating some type of phased implementation and pricing agreement.

According to the Cuban news agency Prensa Latina, that agreement was signed in Moscow on Saturday. While the news agency reported that the accord included protocols for new prices and terms of payments, it gave no specifics.

The Soviets, in cutting fuel deliveries, have eliminated one of the largest sources of Cuba's foreign exchange earnings: the re-export of Soviet crude. Cuban revenue from the re-exports is said to have fallen from $621 million in 1985 to nothing in 1990.

Party leaders in Moscow have said that Cuba will not be set adrift, but a clamor among some legislators and the press there has built pressure in the Kremlin to reexamine aid to Cuba. Particularly vexing to the Soviets is the Cuban debt to Moscow, estimated at $7 billion to $25 billion, depending on the rate of exchange used for the ruble.

The resentment in Moscow has bubbled up recently in a number of vehemently critical articles in the Soviet press, including a dispatch in October in Komsomolskaya Pravda that reported Castro had 32 houses, a personal security contingent of 9,700 and five children by his "current wife."

The last item in particular was stunning. As far as the Cuban public knows, Castro's marital status last changed 35 years ago when he was divorced.

"The word to define Cuba's relationship with the Soviet Union is uncertainty," Cuban Vice President Carlos Rafael Rodriquez wrote in a recent essay in the magazine New Perspectives Quarterly. "We don't know what will happen in the long run. The only certainty is that, in the immediate future, the situation will worsen."