Voices on the left in this country have protested U.S. aid to the Salvadoran government. The brutal murder of the archbishop of San Salvador, the killings of three American nuns and the assassinations of two Americans employed by the American Institute of Free Labor Development have intensified the outcry. (Knowledgeable people in the AFL-CIO, which supports the institute, believe these lates crimes were commissioned by right-wingers living in Miami.)

The conflict agonized the Carter administration and promises to do the same for President Reagan. The October 1979 coup, which ousted the old military-led, government, was welcomed by the Carterites, for it gave the promise of improvement in human rights.Salvadoran Marxists feared the new group's reforms would weaken their appeal; they stepped up their violence and courted non-Marxist leftists to present the appearance of a broad, benign, "popular" front. Left-wing propaganda throughout the world joined in the word war, while terrorist bazookas blew out the secure conference room on the top floor of the U.S. Embassy.

The State Department's dilemma became acute when the nuns were murdered. All aid was suspended while a special commission went from Washington to investigate. The conclusion was that undisciplined national guardsmen, acting on their own, may have been responsible. Aid was resumed, and FBI agents were dispatched to assist Salvadoran police.That the Carter administration would provide police assistance is, incidentally, ironic. Prodded by human rights activists, the United States years ago ended help to Latin American police for fear they might engage in torture. The current reversal has not evoked a squeak from those who earlier would have raised a hue and cry.

Basic decisions will confront Reagan. He is publicly on record as saying that the present government offers the only promising alternative. The president of the ruling junta is a longtime Christian Democrat, and, although the left may label itself a "popular front," there is little sign that the populace supports it.

It seems clear that the Sandinista government in Nicaragua is tilting toward left totalitarianism and that it might, like Moscow's Cuban lap dog, become the next base for Soviet adventurism. That El Salvador should follow suit is plainly intolerable. Great power spheres of influences are more than textbook verbiage; to let significant parts of Central American and the Caribbean fall to our enemies would proclaim our abdication from superpower status.

On the other hand, to sit by and watch brutal rightist authoritarians take over would be equal folly, Salvadoran refugees who finance the murder of Americans should be expelled. We should never forget the recurrent pattern: corrupt and oppressive rightist regimes beget dangerous and oppressive leftist regimes. Cuba itself, Grenada, Nicaragua an perhaps El Salvador make the lesson plain. Often the debate in this country poses a false choice between extremes: if we don't help the Somozas, the communists will take over. The point is that within our spheres of influence we should strive to see that Somozas do not arise. A policy of strong support for democrats would obviate the frequent choice between Scylla and Charybdis.

Last month I lunched with two friends visiting from El Salvador, a lawyer and a banker. Neither is a member of the wealthy "14 families" (most of whom have fled the country), and both have chosen to stay in spite of clear danger. I asked why."Because," the lawyer replied, "we want to see our country a stable democracy. By staying, we can help bring that about." Given U.S. military and development support, they and others like them might just succeed.