Ronald Reagan doesn't become president until Jan. 20, but in respect to at least one burning foreign policy issue, El Salvador, there is strong reason for him to make some kind of statement now. It would put him into the middle of a moving situation even before his policy team, let alone his policy, is in place. But just by being elected, he was thrust into the middle of that situation. If he waits to join the issue until he is in the White House, the situation may well have evolved to the point where his best choices have gone by the board.

The emergency arises out of the massacre last weekend of the top leadership of the leftist Revolutionary Democratic Front. The perpetrators apparently were from the fanatical right, a segment of society that, along with the equally fanatical left, has turned El Salvador into a charnel house over the past year -- some 8,000 people have been murdered. The evident purpose of this particular slaughter was to consummate the task, embraced by both the far right and the far left, of precipitation an all-out civil war. The Carter administration has been struggling to help the all-too-flimsy center hold.

Ronald Reagan, of course, has given no encouragement or support whatever to the viciousness practiced by either Salvadorean extreme. His stated and presumed favor for anti-communists in revolutionary situations, however, has been widely noted in El Salvador and has contributed to the political brew there -- and elsewhere in Latin America. Some elements of the right have hoped to enlist the Reagan predisposition in the service of their own unprincipled grab for power. It is to be expected that these elements will do everything in their power by Jan. 20 to blow the civil strife there into an inferno that would seem to leave the new president no alternative but to back the side purporting to stop the tide of communism -- as corrupt and discredited as those elements in El Salvador are. It was to warn of this looming calamity that a respected civilian member of the Salvadorean junta, Jose Napoleon Duarte, quietly visited Reagan aides, as well as President Carter, a few days ago.

The junta is a gamble. Along with its reformist elements are repressive forces with links to the far right. It represents, however, what chances remain in El Salvador to build a barrier against communism and fascism alike. rThere is nothing else. What the American interest now requires is an indication by President-elect Reagan that El Salvador cannot go back to the pre-junta brutality and injustice and that he is as opposed to terrorism on the right as to terrorism on the left. That would not solve the problems he will still have in El Salvador after Jan. 20, but it will leave him some options.