IN EL SALVADOR, whose civil war makes Nicaragua's look like a tea party, the answer has always been to move the conflict from the military arena to the political. So deep is the inclination to violence, however -- especially violence that spills over on civilians -- that progress has seemed beyond reach. Nonetheless, the Arias peace plan has produced electrifying events in El Salvador in recent days.

The two leading civilian politicians of the left have returned to San Salvador to join the political scene. These are brave men: Guillermo Ungo and Ruben Zamora were driven into exile in 1980 by right-wing death squads and face new death threats now. They are also moderates who earlier belonged to the very same small democratic political community inhabited by President Jose Napolean Duarte. As the two returned, Mr. Duarte demanded that they break their links with the military wing: ''They have to define themselves. Either they are for the democratic process or for violence and guerrilla war.'' But of course by returning they do define themselves: they are for the democratic process, with Mr. Duarte. Peace in El Salvador will be measured by whether they find a political opening -- more basically, whether they escape being murdered.

Meanwhile, Mr. Duarte reports a breakthrough in bringing to justice the killers of Archbishop Oscar Romero, who was gunned down while saying mass in 1980. This assassination came to represent the pervasive terrorism of the military-allied right wing. Mr. Duarte's inability to hunt down the killers has been a heavy burden on democratic civilian rule. Now, a prime (Salvadoran) suspect has been arrested in Miami, and a government witness says the murder was planned by Roberto D'Aubuisson, a presidential candidate in 1984.

The anticommunist cause in El Salvador has been deeply compromised by the democrats' need to accept an alliance of convenience with powerful and corrupt feudal-military elements. In the return of democratic opposition figures and in the Romero case, a dramatic confrontation has been joined between the two uneasy partners in that alliance. Never was there a more important moment for the United States to convey that it supports all of those prepared to embrace democracy in El Salvador, and none of those prepared to destroy it, whether on the left or the right.