When the President of the United States declares that huge increases in military and economic aid for El Salvador and other Central American countries are needed because the ''US national security'' is imperiled in the region, his comments cannot be lightly ignored. But this does not lessen a growing public concern that the United States is again becoming bogged down in an unwinnable civil war abroad.
As the Congress takes up Mr. Reagan's request for enlarged aid, especially for training of Salvadorean troops, these are some questions to be asked:
* Is there evidence that the Salvadorean Army troops are prepared to fight the leftist guerrillas in the manner advocated by their US mentors? How well have the Salvadorean troops trained in the US in late 1981 and early 1982 performed in battle conditions? If the pro-government forces won't fight for their cause, there obviously is a limit to how much the US can help.
* Do the guerrillas have wide popular support, a factor which would militate against too much more aid to the government side? There is evidence they do not. Inasmuch as Salvadoreans tend to be conservative in their traditions and, to some extent, in politics, this would be a factor on the side of helping their present government.
* On the other hand, is the right-of-center Salvadorean government doing enough socially and politically to win hearts and minds? If not, is this merely a matter of a lack of money - or a lack of political will? It is hardly worth propping up a government that is alienating citizens by continued abuse of human rights and failure to carry out social reforms.
* How valid is the ''domino theory''? Many believe Guatemala would not be far behind if El Salvador fell to Marxist rule. Nicaragua already is fast moving far left. But does it follow that Costa Rica and Honduras, with established democracies, and Mexico, with stable institutions, would inevitably go Marxist - anymore than all the countries of Southeast Asia did after the debacle of Vietnam? Or that communist regimes would want to be under the thumb of the Soviet Union?
Of course the United States has a strong security stake in Central America. Defense problems obviously would arise if the US had Marxist governments on its border (though the problem may not be as critical as the President says). Even disregarding the security issue, the US should want to help the people of the region stay free of a system that promises only more repression. If there is a reasonable chance to stop the inroads of Marxism, moral considerations alone demand that the effort be made.
However, the dilemma for the US today is that it seems to have found its conscience rather late in the day. It cannot escape the fact that its misguided policies of the past - its support for oppressive rulers and regimes and its apathy to social and economic reform - have contributed to today's revolutions. If Washington twenty or even ten years ago had launched the kind of ''Marshall Plan'' UN ambassador Jeane Kirkpatrick pleads for now, the picture might have looked quite different.
The stark question is: is it too late? Perhaps not, and we hope not. But, with the El Salvador government's position having deteriorated on the battlefield, the only realistic way out of the dilemma seems to lie in peace negotiations - as Pope John Paul II urged during his recent visit to El Salvador and as a growing number of US lawmakers also ad-vocate.
Certainly the hefty boost in military aid the President wants this year (another $110 million) should not be granted without conditions, including talks with the rebels to bring them into the election process, better performance by the Salvadorean Army, and pro-gress on human rights and on land and other reforms. The fact that Costa Rica and others in the region are trying to meet soon - without US participation - to find a negotiated way out should lend weight to congressional insistence on peace talks.
As for economic assistance, it is good to hear Mr. Reagan saying that ''bullets are no answer to economic inequities, social tensions, or political disagreements.'' A strong program of help for Central America as called for by Mrs. Kirkpatrick should be the centerpiece of US policy. If the United States, after years of shortsighted neglect, can persuade impoverished Salvadoreans and others that it is as interested in their well-being as it is in ''US national security'' (and its security, after all, lies in their well-being), perhaps it is is still possible to turn, or at least effectively contain, the tide of Marxism.
It's worth a try.