U.S. policymakers trying to juggle the continuing political crisis in El Salvador have recurrent nightmares of a civil war there that will lead to full-scale insurrection throughout Central America.

In one version of this scenario, a victory by the Salvadoran extreme left further incites the incipient revolution now under way in Guatamala to the north, and undermines the tenuous stability of Honduras in the east.

In another, a rightist victory in El Salvador, possibly with the help of Guatamala, leads to concerted right-wing military action against the still fragile socialist leadership in Nicaragua.

As one U.S. official put it, harking back to the rhetoric of Vietnam, "the dominoes fall in every direction in Central America."

Tiny El Salvador is the domino in the middle. The United States has tried to keep it from falling by supporting the existing government's claims of ideological moderation.

But the middle ground in violently divided El Salvador has always been a precarious place to stand. Over the past week, many previously moderate elements have begun to choose sides, and the political terrain occupied by the United States and the ruling civilian-military junta it supports has become narrower and shakier.

On the right, the Broad National Front, which many there believe is allied with right-wing paramilitary forces responsible for much of the brutal violence in the country, is moving to consolidate its position with the rich landowners, businessmen and the conservative faction of the military who traditionally have run the nation.

On the left, the three major militant coalitions of workers, peasants and students took a step torward unity in January when they formed the Revolutionary Coordinating Committee of the Masses, known as the Coordinadora.

Last week, in one of the most dramatic recent developments on the Salvadoran political scene, the Coordinadora was endorsed by a newly formed Democratic Front.

Professionals and technicians, previously nonaligned unions, the social democratic National Revolutionary Movement and dissident Christian Democrats who have opposed their party's participation in the junta are all part of the Democratic Front.

For the United States, one essential problem is that few Salvadorans believe that the middle it supports is in fact the middle.

A de facto policy of "reform and repression," as it was termed by assassinated archbishop Oscar Romero, has won few friends and fostered countless enemies for the current government and its U.S. backers.

The junta, meanwhile, has been unable to win significant support within the country's conservative private sector or on the moderate left.

The junta "wakes up in the morning and all they can think about is how they're going to stay alive," one diplomat said. "They can't really negotiate with anybody."

One key to the problem is the armed forces.

The first progressive revolution in El Salvador's modern history began Oct. 15 when, with U.S. approval, young military officers seized power from the corrupt dictatorship of Gen. Carlos Humberto Romero.

Although many had high ideals, after years of military repression the officers found it almost impossible to form working alliances with reformist civilians. The level of suspicion and distrust was too high.

The first coalition government, which included many of the people now in the Democratic Front, distintegrated at the end of last year after a series of political power plays by some of the remaining conservatives in the military's high command.

The Christian Democrats filled the gaps in the junta in January, supposedly on the conditions that sweeping reforms would be made, the military brought under control and talks initiated with the left.

Under heavy pressure from the United States, major economic and land reforms were begun in early March. But the current junta maintains that the left is intransigent in negotiations and the best the current Christian Democratic members of the government can say about the armed forces is that they are "in a process of controlling" the "excesses" of elements within their ranks.

"Excess" -- the term used both by the junta and U.S. diplomats in El Salvador -- is taken to mean massacres of peasants who are not directly involved with guerrilla groups.

The young military officers who supported the October coup find themselves in a difficult position. They have received little or no thanks for the reform they sponsored and the "excesses" of more powerful factions within the Army and security forces tend to discredit the entire military.

Some leftists hope that if full-scale civil war breaks out, the armed forces will divide. However, many young officers who say they support reform also say there is no way they could side with the people who have been shooting at them for all these years.

"We cannot crawl into bed with the left," one young major said. "We would have to go with the right. It would be the end of everything."

All sides in El Salvador are studying closely the example of Nicaragua. The left in particular is trying to follow a diplomatic course similar to that of the Sandinista National Liberation Front, which was able to win broad domestic and international support for its successful war against the Somoza dictatorship in Nicaragua.

El Salvador's revolutionaries are finding, however, that their task is much more difficult. There is no single enemy such as Somoza and his National Guard to fight and their own ranks remain more divided than those of the Sandinistas.

On the foreign front, Cuba, while generous with advice, apparently has not provided substantial material support.

A major in Salvadoran intelligence, asked about allegations of Cuban involvement made by some U.S. officials, replied: "We think the Cubans are sending in some arms. We have heard they are. But what we can't figure out is where these arms are."

Panama's strongman, Omar Torrijos, was a key supporter of the Sandinistas. But, although he backs reforms there, Torrijos went to school in El Salvador, knows many Salvadoran officers personally and is not expected to give substantial support to a left-wing cause that makes its first priority the trial and punishment of his old schoolmates.

Venezuela, which also helped fight Somoza, now has a Christian Democratic rather than a Social Democratic government that supports the current rulers in El Salvador.

Meanwhile, there is mounting pressure on Mexico to break relations with the Salvadoran government. Such a move by Mexico, according to Christian Democratic junta member Napoleon Duarte, would create "chaos." The Mexicans have yet to announce their decision.

The right, meanwhile, is looking for support from conservative governments in neighboring Guatemala and Honduras.

While the rest of the world ponders the situation, the United States remains the only firmly committed friend of El Salvador. With its diplomatic and economic power, it still hopes for a middle-of-the-road solution to stop the "advance of communism" without the moral cost of supporting a ruthlessly oppressive dictatorship.

U.S. diplomats repeatedly have ruled out the possibility of sending in troops, and although military aid to the junta is on the way, it will not include once-contemplated military advisers.

As El Salvador's battle lines become more sharply defined, however, many observers are now questioning whether the United States might not continue to support the middle until no middle remains.