The Reagan administration faces a hard fight in gathering congressional support over the long haul for its plans to increase aid to El Salvador.
Current support might be sufficient to approve an expected supplemental economic aid request from the administration. But a good part of that support appears to be reluctant, hesitant, or tentative.
Despite extensive testimony by administration officials, many senators and congressmen - from the right, left, and center of the spectrum - clearly have their doubts about the way the administration is proceeding in Central America. Many also have come to realize that the choices in El Salvador may come down to picking among unpleasant, or as one staff aide put it, ''unsavory,'' alternatives. One Republican congressman who has supported the administration's Salvador policy throughout now is described as ''nervous and uneasy.''
Anxiety on the part of a number of congressmen derives from a combination of factors: (1) a feeling that the administration does not yet have a coherent and farsighted Central American policy, (2) a feeling that the administration may, in some cases at least, be misleading the Congress as to the human rights situation in El Salvador, and (3) a feeling that most congressmen do not have the background needed to judge the Salvador situation. On top of all that, it is an election year, and a year in which overseas military aid is being increased while domestic programs are cut. As the aide to a congressman put it, ''There are certainly no votes to be won in El Salvador.''
Pervading the atmosphere is the ghost of Vietnam. Although not all senators and congressmen say so publicly, the specter of step-by-step involvement in El Salvador analogous to the Vietnam involvement looms in the background. The Salvador and Vietnam situations are clearly different in many regards, but the Vietnam comparison will not go away. An American Army trainer in El Salvador seen carrying an automatic rifle has been recalled to the United States. A reported CIA plan to support secret operations against Nicaragua may add to congressional nervousness.
Given all this uncertainty, some senators and congressmen who support the administration on El Salvador are urging that President Reagan devote a major public speech to the issue in order to generate greater understanding and support. Indications are the President will address the issue within the context of ''positive and forward-looking'' proposals for the economic development of the Caribbean basin.
So far, the politicians have little to go on in the way of public opinion polls. A Gallup poll last March showed that 2 of every 3 ''informed Americans'' feared that the situation in El Salvador would develop into ''another Vietnam.''
But the Reagan administration came to office indicating it considered the ''Vietnam syndrome'' to be part of the past. Certainly since the mid-1970s, an increasing number of Americans have come to favor increased defense spending. But when it comes to entanglements in murky third-world situations that might at some point involve US troops, many Americans clearly still have their doubts. Administration officials, meanwhile, insist that the use of US troops in Central America, while not ruled out as an option, is not under active consideration.
What worries some Americans are press reports which indicate that widespread killings of unarmed civilians have been carried out by the government's security forces or by right-wing death squads associated with them. President Jose Napoleon Duarte appears to be a decent man, but there is doubt that he holds much power in comparison with the country's military leaders.
''Presumably there are more good guys on our side than on the other side,'' said a State Department analyst. ''But there are really no clear-cut good guys. . . . Ambiguity of this kind is something that Americans don't like. . . . If they can't identify the good guys, they want out.''
Some of the Americans who think in this way have been expressing their concerns in the form of letters and post cards to their senators and congressmen. Some have been mobilized into letter-writing campaigns on behalf of groups opposing US military aid to El Salvador.
A check with several congressional offices showed mail running heavily against further US involvement in El Salvador. Congressional aides said that this was to be expected because protesters are always the first to react to controversial issues. But some congressmen admit that it does have an impact, particularly in the absence of countervailing views from their districts.
What keeps congressional support behind President Reagan in the face of all this? To begin with, concern that there could be a Cuban-supported Communist takeover in El Salvador, say some congressional sources. There is also a reluctance, they say, particularly on the part of Republicans, to oppose a still popular President.
Some Democrats have been outspoken in their opposition to further military aid to El Salvador. But they alone will not be able to stop the administration. It is the Republicans who must be watched. A good number of Republicans are offering the President tentative support but are at the same time trying to keep hands off until they can see which way the wind is blowing. To be more precise, they are waiting until they can see which way some of the better-informed Republicans will move.