FOUR weeks of post-election sorting out in El Salvador have left the truly dubious hero of the right, Roberto D'Aubuisson, as president of the new constituent assembly, or legislature--his coalition won 36 of its 60 seats. The newly designated president of the government, Alvaro Magana, who will serve until presidential elections next year, is something else. A banker trained as an economist at the University of Chicago, he comes from one of El Salvador's "14 families" and early on embraced the need for change. He was chief of tax reform at the Organization of American States in the Alliance for Progress. An independent favored by the Christian Democrats, he is regarded as a conciliator and pragmatist with excellent contacts among Salvador's democratic opposition. His selection has angered the hard right.

The way Mr. Magana was selected is important. His was one of three names on a list that the armed forces, intent on having the civilian politicians act in the progressive spirit of their 1979 coup, put before the political parties. American diplomats had warned that American support would hinge on the choice of a representative reformist government. A full deck of American political figures seconded that motion, not least Jesse Helms, who sent an aide to tell the Salvadoran right not to look to the American right for indulgence. It amounted to a substantial American intervention. It's awkward, but it was made necessary by the considerable investment the United States has made in the future of El Salvador. The point is to do it well.

In the political phase opening now, the government and the more conservative assembly will likely differ on the reforms. Each may appeal to the United States. The administration will be under pressure to voice its private ideological and practical doubts about some of the reforms. But it cannot forget that maintaining political momentum, by supporting the reforms, must be the first priority.

Sharing that billing is the urgent need to stop killing civilians. Just the other day, peasants in one village reported that the army had killed 48 or more people. Of President Magana's intent to halt this slaughter there can be no doubt. He will need all the help he can get to induce the military leadership and the armed right to throw their full weight against it. Will he get help from Mr. D'Aubuisson, a former officer known for his links to the unreconstructed oligarchy and its death squads? A D'Aubuisson supporter now says, referring to American plans to link aid to reforms and human rights, "It's just a complete bluff." He is wrong about that, and Mr D'Aubuisson will make a terrible mistake not to understand how much.