IN THE SEETHING cauldron of Central America, the United States is being called on because of its power and its interest to help guide a process of revolutionary change. This would be a tricky role in the best of times, but at this moment, when events and the election campaign have magnified apprehensions of disaster on our doorstep -- of "another Cuba" -- it is a thankless task.

The toughest current choice is in El Salvador, where a well-meaning but pitfully weak civilian-military junta is struggling to impose order on a landscape wracked by violence of the left and right alike. On the unassilable premise that order is the first priority, the administration has been modestly trying to train and equip the police and low-level military forces. Not only is the program having little visible effect in dampening the violence. The program, and reports (denied) that the United States intends to send Army advisers and $7 million in equipment, have raised the politically explosive specter of an actual intervention. The bolder -- and sillier -- assistance proposals seem to represent only some Pentagon quarters' pipe dreams. Still, it can't be said too often that any American plans drawn to help a Central American government keep domestic security must be developed in ways that do not worsen the atmosphere they are supposed to calm.

If the aim of American policy in El Salvador is to steer a revolution still in process, in Nicaragua it is to guide one that has already taken place. In the new aid bill, Congress is being asked to support an administration judgment that the regime in Managua, though it leans left, is not altogether lost to pluralistic democratic rule. Overhanging this gamble is everyone's recollection that 20 years ago in a similar context the United States guessed wrong about Fidel Castro's Cuba. A new $75 million aid package is meant to show Nicaraguans they have an alternative to the Cuban model, and to meet the particular credit requirements of the middle class. Many entrepreneurs are deemed to be waiting for just such an American signal before committing their own resources and energies to the struggle for an open society.

The common thread of American policy in Central America is to accept the revolutionary context and to try to preempt the elements and openings favorable to Cuba by supporting the forces of the non-communist center-left. This represents a historic change for the United States, for decades the bulwark of the reactionary status quo. In its various parts, however, the region is abandoning that status. As difficult as it is to judge the currents, it makes sense for the United States to go with the flow.