HANS-JURGEN Wischnewski, the West German figure who follows these things, has been trying to build a negotiation in El Salvador, and again Mexico and Venezuela are seeking to launch a peace initiative. Dr. Wischnewski, whose party, the Social Democrats, is much enamored of the El Salvadoran left, seems a long shot. Mexico and Venezuela, especially when taken together, have impressive credentials. They have access -- Mexico leans to the opposition and Venezuela to the junta -- and a direct interest in a peaceful outcome. They evidently agree on the Zimbabwe precedent of external mediation followed by elections.

Concerned outsiders would be doing less than their duty if they did not try to open up a negotiating path. It does not seem farfetched, moreover, that the civilian leaders of the parties, Napoleon Duarte for the junta and Guillermo Ungo of the opposition, might make up: They ran on the same ticket in 1972 and served in the government together as recently as 1979. The relevant question is perhaps whether either of them could carry their respective military partners. An answer requires more probing.

There is a tougher question for the United States: Why not support negotiations? The administration suggests that the call for negotiations is a tactic designed to embarrass the junta, whose civilian members at least now pin their claim to legitimacy on elections scheduled for next year. To some extent, the call for negotiations is precisely such a tactic. But it is not only that. The administration should not get in a box where it would be encouraging the junta to impede negotiations largely so that the United States can win a splashy victory over international communism, assuming that is feasible. There is a fine line between forcing the junta into negotiations that its more moderate members would resist, and, by careless American support, inviting the hard-line members to resist negotiations that the moderates might prefer. In any event, if the United States is to stay on the elections track, it must ensure that junta-run elections will be free and fair enough to win general respect -- a tall order.

In the interim, the administration has an urgent task that, it insists, gives it no qualms: get the junta to control the death squads operating under its right wing. Some progress in pressing investigations into the deaths of Americans and others has been reported, but it is only a taste. This is the single issue on which the play of forces within El Salvador most critically hinges. It is the issue on which congressional support of the administration is, precariously, balanced. Cut ring-wing terror and the whole prospect in El Salvador will change.