On Jan. 10, before the United States sent 51 military trainers and millions of dollars of military equipment to El Salvador and drew a line daring the Communists to cross, the guerrillas launched their "final offensive." It fizzled. Would that it were final. Nine months later, the guerrillas have grown stronger and capable of military operations they could not perform a year ago.

To its credit, the Reagan administration has urged the military to end the repression, but having drawn the line earlier and untied aid, it discarded its leverage. The administration talks about a political solution, but the security forces, now acting with impunity, pursue a military solution. The Salvadoran government, however, cannot win by force because of its relative weakness and because of the demography. With over half the population under the age of 17, Salvador's youths are not intimidated when family and friends are murdered by security forces; they're politicized.

The security forces have become their own worst enemies; they may think they're killing the left, but they're actually recruiting for them. And the United States is the accomplice, however unwitting.

The current policy is leading inexorably to more repression, a stronger left, and deeper involvement by the United States Those who argue for escalation would only reach disaster more rapidly. They should ponder why the guerrillas became weaker in 1980, when the United States was not providing lethal military equipment to the government, and stronger in 1981, when we were.

The administration is correct to support elections; they remain the best way yet devised for testing the will of the people. But they are meaningless in the current climate of violence. If they are held, they will not legitimize the government. Indeed, if rightists ally with certain elements of the security forces and rig the elections, they could destroy the only coalition capable of moving El Salvador eventually to democracy. The administration insists that the left give up its weapons and accept elections before negotiations can begin, but what does that mean when the military keeps its arms?

This is a moment to act. The government still is in a position of strength, and the left is ready to talk, having just dropped its preconditions.

The United States must encourage the military to drop its precondition that the left give up its arms, and allow President Duarte to negotiate with his counterpart on the left--and with moderate businessmen on the right. Instead of condemning Mexico and France, we ought to encourage them and the Venezuelans and others to provide a framework for talks, which will strengthen the hands of the moderates in both camps.

By refusing to talk, the government keeps the more heterogeneous left united. The idea that negotiations can only be manipulated by the left for its ends is a confession of incompetence. Unless the United States puts its muscle behind talks, they won't happen.

The agenda of negotiations is not how to share power, as some who oppose negotiations suggest, but rather what can be done between now and March 1982 to make the elections to the constituent assembly free and fair. Even the Christian Democrats would be foolish to participate without such guarantees. If, on the other hand, the left is given the opportunity to negotiate and doesn't, who can defend it?

Negotiations should start with confidence-building measures--perhaps cease-fires in certain areas under international supervision. The left will want to restructure the armed forces and eliminate repression. That is also in our interest, and in Duarte's interest. But we ought to extract a price for it: the French and Mexicans should induce the left to participate peacefully in elections and to disarm their most repressive groups. Any step in that direction will be an improvement, and moderates at least will be calling the shots while talks occur. Unfortunately, by allowing our military attach,es to negotiate arms aid directly with their counterparts, instead of giving Duarte the power to turn aid on and off, the United States, too, is assisting the extremists.

The United States should also encourage speeding up the land reform. If the conferring of titles to the land is completed by March, the campesinos will have a stake in seeing that the election is not stolen.

An amendment before Congress conditions future aid to El Salvador on progress in negotiations, land reform, and reducing repression. The Senate has passed it. But the administration need not wait for the House; it can act now to implement its intent, to retrieve the leverage it discarded last January and to pursue negotiations, not obstruct them.

If we don't put such conditions on aid soon, we will find ourselves captive to a creeping, self-defeating rightist strategy, which will present the administration with the choice of either erasing the line it has just drawn or fighting an unsustainable and unwinnable war.

We cannot expect the Salvadoran government to be grateful for such a public display of leverage, but it is necessary for the American people to indicate publicly to El Salvador's military that our support is not a blank check and that it will cease if terrorism is condoned.