Senior administration officials, when asked how far President Reagan might go to achieve his policy goals in Central America, are apt these days to quote the president's frequent promise that "we're not going to lose a country to communism on our watch."

"The president is going to do whatever is reasonably necessary to assist the democracies in Central America," a high administration official said last week. "That's a big menu."

Pressed to be more specific, this adviser listed such possibilities as more U.S. advisers to train troops in El Salvador and Honduras, more airstrips and medical teams to assist these troops, more political pressure on the leftist Sandinista government in Nicaragua, more use of sophisticated aerial reconnaissance, more undisclosed contingency plans that could deal with everything from a full-scale war in Nicaragua provoked by the anti-government and U.S.-backed guerrillas to a Nicaraguan invasion of a neighboring country.

And another adviser, after a gloomy survey of military prospects in El Salvador and review of the military buildup in Nicaragua, offered an observation rarely made explicitly, at least to reporters. "You could conceive of the circumstances," he said, "where not losing a country to communism means having to take one back."

This adviser and others deny that there is any master plan for invading Nicaragua and bringing down the Marxist government there. They said there was no truth to a rumor of last week that the U.S. government had an emergency plan for a "surgical strike" on the Nicaraguan capital of Managua.

But these senior administration officials acknowledge that there is a wide range of contingency plans that could deepen U.S. military involvement in Central America. Many of them see a widening war in the region. And all of them say that Reagan's belief that Central America is vital to U.S. interests is a key to understanding what would occur in a crisis. If a U.S. ally is in danger of going under, said these advisers, Reagan would not back down in Central America regardless of the domestic political consequences.

"He won't shrink from the tough decision," said one senior official. "Stopping communism in the hemisphere is a matter of deep conviction to the president."

Another official said: "Everybody at the White House feels you've got to do what has to be done down there. It's a matter of vital U.S. strategic interests."

These comments came against a backdrop of stepped-up U.S. planning aimed at providing the airfields, roads, bridges and medical support that would enable the United States to intervene in a crisis. Though the word "infrastructure" is out of vogue these days because of its association with Vietnam, one administration official said this meant that the United States would "have the necessary means to become heavily involved if we had to" in Central America.

On the surface, this preparation might seem contradictory to Reagan's oft-repeated pledge to send no U.S. combat troops to El Salvador. But officials say Reagan means exactly what he says and wrote this promise into the speech he gave to a joint session of Congress on April 27 after it was omitted from the original draft.

These officials say Reagan is convinced the Central Americans can do their own fighting, if given the proper training and support. In any case, they add, he has always been critical of large-scale commitments of U.S. ground troops to long inconclusive wars, which could mean that he would favor a decisive, quick strike aimed at the opposition in an emergency.

Reagan's political history on the subject is instructive. He first started supporting Republican candidates during the Eisenhower years in the 1950s when it was an axiom of conservatives that it was a mistake to "get bogged down in a land war in Asia."

During the Korean and Vietnam wars, Reagan believed in striking at the source of the aggression. When he was a candidate for president in 1976 and again in 1980, one of his most frequent applause lines was the declaration: "Let us tell those who fought in that war that we will never again ask young men to fight and die in a war our government is afraid to win."

As the complexities of international affairs have become more apparent to him during his presidency, Reagan has both broadened and softened many of his attitudes. He has become more conciliatory to the People's Republic of China, for instance, muting his long admiration for the government on Taiwan. Pressed by U.S. allies, he backed away from his opposition to selling high technology items to the Soviet Union. Last week he demonstrated new flexibility in proposing a revised treaty with the Soviets to limit strategic nuclear weapons.

But on Central America, Reagan's fundamental anti-communism appears to have been unmodified by the presidency. Nor does it seem to have been affected by public opinion surveys that show widespread public uneasiness over the U.S. involvement and growing fears that El Salvador could become "another Vietnam."

"The president isn't looking at the polls," said one adviser last week. "He's acting out of conviction."

That conviction has been stated by Reagan many times, perhaps most ringingly last March 4 before the Commonwealth Club in San Francisco where the president responded to a question by saying "there is no parallel whatsoever" between El Salvador and Vietnam, and then blamed "the Iron Curtain countries" for fomenting the violence in Central America.

"The threat is more to the entire western hemisphere and toward the area that it is to one country," Reagan said. "If they get a foothold, and with Nicaragua already there, and El Salvador should fall as a result of this armed violence on the part of the guerrillas, I think Costa Rica, Honduras, Panama, all of these would follow."

This was a Reagan statement of a "domino theory" first enunciated in 1954 by President Eisenhower in defense of his decision to send economic aid to the South Vietnamese government.

"You have a row of dominos set up," Eisenhower said at a news conference, "you knock over the first one, and what will happen to the last one is that it will go over very quickly. So you would have the beginning of a disintegration that would have the most profound influences."

This could well be a statement of Reagan's views about Central America. Moreover, it is known to be the unanimous view of senior policy advisers who frequently differ on other issues. On the question of a hard-line Central American policy there is no known difference between national security adviser William P. Clark, White House counselor Edwin Meese III and chief of staff James A. Baker III. And Langhorne A. Motley, the new director of the State Department's bureau of inter-American affairs, is thought likely to be more responsive to the White House than was Thomas O. Enders, whom he replaces.

Central American policy now is directed by a consortium that includes the president, Motley as the surrogate for Secretary of State George P. Shultz, Clark, CIA Director William J. Casey and U.N. Ambassador Jeane J. Kirkpatrick, considered the hardest of hard-liners on Central American issues.

"Every member of this group believes in standing firm on Central America," said one official.

What voices of warning there are in the administration have come from the military, still burned by the experience of Vietnam. The Joint Chiefs of Staff are said to be reluctant to be drawn into step-by-step involvement in the region, and military officials have warned repeatedly that the aid being sent to El Salvador fails to match the administration's policy goals.

Many of the discussions now occurring at various levels of the administration concern what the U.S. "crisis reponse" should be in contingencies arising from the present involvement. These discussions range from what the United States should do in emergencies--such as the seizure of an embassy or a Nicaraguan strike into Honduras--to what many see as the more probable circumstances of slow disintegration in El Salvador following a polarizing election and military successes by the well-trained leftist guerrillas.

Among the "what if" possibilities is the question of what the United States should do if the anti-government guerrillas in Nicaragua carry a terrorist campaign into population centers as their leftist counterparts have done in El Salvador. One official said a proposal to recognize the "contras" as a legitimate government surfaced earlier this year and was rejected. Like other dismissed contingencies, it could be revived.

Officials are reluctant to talk about the U.S. options. Some are worried that publicizing such discussions would give the appearance of planning for a wider war that the administration does not want, and make it harder to get administration aid requests through Congress.

Nevertheless, there is a growing belief in the administration that the Central American conflict is likely to widen and that Reagan, before the end of his term, may be confronted with a situation calling for decisive action, including direct measures against the Sandinista regime.

"No one is looking for a broader conflict," said an official who acknowledged these discussions. "But the president isn't going to lose El Salvador. He isn't going to back down. He really means it when he says that the United States has a vital stake in what happens in Central America."