Eleven months after Secretary of State Alexander M. Haig Jr. vowed to "draw the line" against communist adventurism in Central America, the United States still has not found a way to end the bloody civil wars and political turmoil in the chronically unstable region.

Despite bolstering by American arms and military advisers, the U.S.-backed junta in El Salvador remains unable to stamp out the insurgency of leftist guerrillas.

A guerrilla bombing attack on the country's main military air base last week appears to have crippled air support for ground operations against the insurgents, prompting the administration to warn that the United States "must be prepared to increase" its military assistance.

The prospect of a new infusion of arms for El Salvador comes when congressional liberals and domestic human rights groups have taken loud exception to President Reagan's assurance to Congress that the regime there is making "a concerted, significant and good-faith effort" to end murder and repression.

The developments are threatening to renew last year's debate about whether the administration is creating a vest-pocket Vietnam in El Salvador.

The administration is not having much success at influencing events in other parts of Central America, either.

Nicaragua's radical Sandinista government continues to follow a leftward course that U.S. officials fear is turning that country into "another Cuba." In Guatemala, escalating warfare between left and right threatens to produce a bloodbath that could make the conflicts in the other countries look like minor skirmishes.

And, perhaps most frustrating of all to U.S. policymakers, Cuba's President Fidel Castro, who is regarded by the administration as the chief instigator and exploiter of these troubles, continues his support of Central America's leftist forces, undeterred by Haig's sporadic threats to "go to the source" of communist agitation in the region.

So far, though, there is no sign that the administration intends to return to Haig's tactics of last November, when he went out of his way to hint at military actions such as a blockade of Cuba or Nicaragua.

U.S. officials say privately it became increasingly clear that military action was not a viable option, that it would be difficult for this country to marshal the resources for a large-scale operation like a blockade, and that such action might in any case have little impact on the problems that the United States faces in Central America.

These officials contend that Haig's high-decibel rhetoric was psychologically useful, making Cuba and Nicaragua uncertain about U.S. intentions and helping focus public opinion in this country and in Latin America on the perceived communist threat to the region.

However, the hints of November that new U.S. "actions" would soon become evident are no longer to be heard.

Instead, U.S. policy toward Central America at the moment is best described as "more of the same" in El Salvador and "wait and see" in Nicaragua and Guatemala.

The essentially static nature of this approach underscores how far the administration has moved from its original aim of making Central America the centerpiece of a grand global scheme for confronting communism.

The administration's first major foreign policy decision after taking office a year ago was to throw its weight behind the junta in El Salvador, putting the Soviet Union and Cuba on notice of U.S. resolve to block communist interference in any area considered important to its strategic interests.

That attitude, summed up by Haig's talk of "drawing the line," is still echoed by senior administration officials in speeches and testimony before Congess.

But in practice the administration has learned, through trial and error, that Central America must be dealt with not as a piece in a global East-West chess game but as a regional problem requiring solutions fitted to its circumstances.

The administration has been forced to come to grips with the fact that the threat of communist takeovers in Central American countries results from poverty, social inequality and long histories of brutally repressive military dictatorship.

The administration has tried to prod the rightist regimes that are its clients in Central America toward political and social reform and greater respect for human rights.

But its efforts, viewed as insufficient by its critics at home and abroad, have had only spotty results. And that, some U.S. officials concede, has had an increasingly constricting effect on Washington's ability to maneuver.

They offer this assessment of where U.S. policy stands in each of Central America's principal trouble spots:

In El Salvador, even before last week's daring airfield attack, the officials admitted that the struggle between the government's forces and the guerrillas was stalemated.

The guerrillas, they say, continue to receive infusions of arms and equipment smuggled through Nicaragua and, largely as the result of training in Cuba, have greatly increased their proficiency at hit-and-run tactics.

On the plus side, the officials claim that the scheduled March elections for a constituent assembly, the first stage in a U.S.-supported process to return El Salvador to democratic rule, show signs of attracting greater public support.

In particular, the officials say, influential elements of the Catholic Church, which had been cool toward the elections, now are openly urging the public to take part.

The U.S. hope, expressed in almost wistful terms by administration officials, is that bringing off the elections in a demonstrably fair manner with significant voter turnout could be a big leap toward putting El Salvador on the road toward political stability.

But there is a nagging fear that the guerrillas will intensify their hit-and-run raids in an effort to disrupt the elections, a fear that appears to have been borne out by last week's attack on the air base.

In its rush to warn about the likely need for more U.S. military aid, the administration left no doubt about how much it is banking on the elections as a way out of the Salvadoran impasse.

On Thursday, the State Department issued a statement charging that the attack "is part of a general intensification of guerrilla activity designed to sabotage the free elections" which the statement characterized as "a criticial juncture in Salvadoran history."

In Nicaragua, U.S. officials believe that the avowedly Marxist, pro-Cuban elements of the Sandinista-controlled government are gaining decisive control and are likely to continue turning the country into an armed camp from which leftist revolution can be exported to neighboring countries.

Meanwhile, the Nicaraguans continue to rebuff U.S. efforts to work out a rapprochement, and relations between the two countries remain severely strained.

However, the officials insist the United States is not contemplating any overt or covert actions against Nicaragua other than to try to isolate it within the region by pointing out the potential threat it poses to its neighbors.

There are signs, the officials add, that this campaign is having some effect.

Three neighboring countries--Costa Rica, El Salvador and Honduras--recently formed a Central American Democratic Community for closer cooperation on political and economic issues. One of the chief motivations for the group, U.S. officials say, was concern about increasing Nicaraguan interference in their affairs.

Potentially the greatest problem in the region, according to the officials, involves Guatemala, where there is a new flare-up of a deadly civil war that has gone on intermittently between the right and left since the 1950s.

In the past, the leftists always were suppressed in violent campaigns, but the United States is concerned that this time, with massive Cuban support, they could seriously challenge the military-controlled government.

If so, the administration would be hampered in trying to assist the government, along the lines of its efforts in El Salvador, because of the Guatemalan military's brutally repressive record.

Although Washington wants to establish a new military relationship with Guatemala, the administration has signaled the regime there that it cannot risk the outcry such a move would provoke in Congress unless Guatemala significantly improves its human rights record.

Although domestic liberals are suspicious that the administration is plotting some backdoor approaches toward military cooperation with Guatemala, U.S. officials deny that any such move is in the works.

Instead, they say, any actions, such as a much-rumored proposal to sell Guatemala helicopter spare parts, are being held in reserve until after a new Guatemalan president is chosen in March and the United States has a chance to gauge whether the change will lead to an easing of the repression there.

"Simply maintaining the status quo in our current relations is not an acceptable solution," says one official. "We agree they've got to improve their human rights record, and if they do, we're prepared to test the threshold for acceptability in Congress of a new military relationship.

"But for the moment, we feel our hands are tied, and we've told the Guatemalans so as clearly as we can."

Another official adds, "That's what is so frustrating about the situation in Central America. You measure progress in inches, and you're never sure whether a positive movement in one area won't be wiped out by a setback elsewhere.

"It wouldn't do us much good, for example, if our investment in El Salvador was to pay off only to find that we're facing a worse threat in a larger and more important country like Guatemala."