I HAVE SPENT the last couple of days laboring under a misconception. I thought it was the critics of the Reagan administration who were confusing El Salvador, one place, with Vietnam, quite another place. I was wrong. It is the administration that has them confused. Their light at the end of the tunnel turns out to be downtown San Salvador.

You can understand my mistake. It is the liberals, after all, who are suppose to see a Joe McCarthy behind every tree, a Great Depression behind every tax cut and a Vietnam behind every attempt to stand up to either the Russians or the Cubans. It is the liberals who are supposed to see what they want and conservative who see what is really there, only this time the stereotypes are reversed.

It's clear why this is happening. El Salvador looks like Vietnam. It is not only tropical and humid like Vietnam, it is small, the people are brownish and its recent history is complex and confusing and lends itself, even now, to instant revision. It seems true, however, that it has been ruled by an oligarchy of wealthy landowners and it is true that there is some sort of indigenous war going on, but it is also true -- es verdad? -- that the Cubans and others have taken sides in this struggle and are smuggling arms to the Marxist guerrillas.

Much of that was true in Vietnam. It, too, was an internal struggle waged with lots of external arms. It is also true that our side there, like our side in El Salvador, is not quite pure.The junta is relatively moderate and engaged in land reform, but it is afflicted with the sin of coming upon that solution late and of not being able to control its own police apparatus. Give or take a few thousand indiscriminate deaths, it is not doing a bad job.

Now a Socratic Martian, thrown into this situation, might ask some embarrassing questions. He might wonder, for instance, why the United States has some sort of divine right to supply arms and material to one side of the Salvador fight, but the Cubans do not. A glance at the map would show that the Cubans are closer to El Salvador than we are, and that they are, at the very least, Spanish speaking. That ought to count for something.

The Martian might wonder, too, why it is that we always wind up taking the side of the rich, why a nation founded in revolution always feels more at home supporting right-wing juntas, but mostly he would wonder why it is that El Salvador has become such a cause -- one maybe worth dying for. That, after all, is the inevitable result of what will happen to Americans when they go there as military advisers. Military advisers have a tendency to get killed. It is, as they say, an occupational hazard.

Here the Martian would be stumped. No Martian could say why we have escalated the fight in El Salvador to a matter of national pride. To answer that one, it takes an American who was around during the Vietnam war and who has listened to the campaign rhetoric of Ronald Reagan and the remarks of his secretary of State, Alexander Haig. They both subscribe to the stab-in-the-back theory of why the Vietnam war was lost. They think America could have won had it not flinched. They think it was sold out -- certainly by the media, maybe even by the people themselves.

It was Reagan, after all, who called Vietnam a noble war and who just recently said that Vietnam could have been an American success, only the military "was denied permission to win." As for Haig, he has never made any secret of the fact that he subscribes to that theory. The war could have been won, he said, had the president been able to "apply the full range of American power to bring about this successful outcome."

So it is not Reagan's critics who see El Salvadonas Vietnam.It is Reagan and Haig who are doing that. It is clear that to them El Salvador is merely a continuation of the struggle begun in Vietnam -- waged for the same reasons, against the same enemy -- only now moved, as they always thought, closer to home. To Reagan's critics, El Salvador just seems like Vietnam. To Reagan and Haig, it is Vietnam.