Once more into the breach

President Reagan wants Congress to give him more money to support the existing but embattled conservative regime in El Salvador and also allow him to continue to support a counterrevolutionary insurgency against the left-wing regime in Nicaragua.

Congress is reluctant.

Behind the reluctance is a collective folk memory of what happened not so long ago in Vietnam, and farther back in other places.

Behind it also is awareness of a disconcerting immediate fact. The regime which the President wants to sustain in El Salvador is shakier and having more trouble on the battlefield today than it was having two years ago before United States aid was put into the scales.

When an operation begins, like the current one to sustain a right-wing regime in El Salvador, it always seems so easy. El Salvador is a small country. Insurgency did not reflect mass support. In theory, giving just a little more muscle to the ruling regime in the form of guns and bullets and military training should end the matter then and there.

But the trouble with the theory is that it overlooks the effects of US intervention on various sorts of people.

For example, in Vietnam a new nationalist movement developed during World War II. The leader was Ho Chi Minh. Ho did not at first look to either China or the Soviet Union for help. He himself was given early help by the American OSS which , after the war, became the CIA. He asked for more US support after the war. The OSS recommended continuing aid to him and to his nationalist forces.

If that recommendation had prevailed, the Vietnam story would indeed have been different. But the recommendation was overruled. Instead of backing Ho's nationalists, the US backed an attempt by the French to regain control over Indochina. Ho had no choice. He turned to Moscow and, later, to Peking for help.

Thus US intervention in Vietnam began as the ally of the prewar foreign, colonial regime. The natural tendency of local nationalists was therefore to regard the US, not as a savior from communism, but as the outside force attempting to refasten foreign colonial rule on the country.

The original decision to back the French rather than Ho Chi Minh's nationalists led straight on into massive American military intervention, which in turn brought counterbalancing Soviet and Chinese help to the nationalists. American intervention induced intervention by others. The US cause in Vietnam was never able to shake itself free from the incubus of having come in originally in support of the French colonial regime.

If the US could intervene in El Salvador without anything else happening, then just a little intervention might produce quick and decisive results. But the US cannot intervene anywhere without inducing counterbalancing interventions from elsewhere.

In El Salvador itself some people who were either neutral or perhaps even supported the regime turned against it when US intervention began. The mere fact of US intervention probably gave a boost to insurgent recruiting and also to insurgent respectability in the eyes of local nationalists.

Also, Nicaragua, Cuba, and the Soviet Union would automatically welcome a chance to frustrate US involvement. Those countries can escalate their help to the insurgents more easily than can the administration in Washington. President Reagan has to sell his case to a reluctant Congress. The junta in Nicaragua, Fidel Castro in Cuba, and the Soviets do not have a similar Congress or skeptical public opinion to restrain them. If they want to help the insurgents in El Salvador, they go ahead and do it.

In other words, US aid to a regime in Central America is not a net advantage to that regime. It is in part canceled or balanced off by resistance to the very idea of US intervention. It is also in part canceled or balanced off by intervention from other outside sources.

An act of US intervention might in fact be more than offset by the negative actions which US intervention induces.

We cannot yet calculate or foresee the end result of the present policy of intervention in El Salvador. We do know that on balance the regime seems to be worse off today with US aid than it was before that aid began.

It is possible that with enough support from Congress the President could put sufficient support into El Salvador to produce a big win for the regime. But how much would be ''enough''?

The danger is that US intervention will win, not a reputable government in El Salvador, but another score for Moscow. This is a high-risk venture.