THE HEAD of the junta's agrarian reform program and two American advisers are the latest victims of the terror that is ravaging El Salvador. Beyond the human tragedy, the political loss is grievous. The agrarian program has been the centerpiece of the American-backed effort to preempt Cuban-supported revolution, and as such it has been fought by the left. It has also been fought by upholders of the traditional, feudal status quo on the right. Having enemies like these is a tribute of sorts to the quite striking, if incomplete, success of the program so far. But it is a costly tribute.As the distribution of land to farmers is further inhibited, the junta's crucial attempt to consolidate a peasant base will be impaired.

Both left and right in El Salvador have stepped up violence in anticipation of Ronald Reagan. The left's guerrillas have launched a "final offensive" so as to present him with an "irreversible military situation" by Jan. 20. They seem not to undersand that nothing could do more to strengthen those voices in the Reagan camp demanding to make El Salvador a test of the new administration's anti-communism. Meanwhile, the right with its "death squads," some with government connections, is blowing on the fire, hoping to induce Mr. Reagan to commit American prestige, arms and perhaps troops to the anti-communist side -- and, not so incidentally. to restore its old privilege by repudiating the agrarian reform.

Some influential Republicans, centering around Sen. Jesse Helms, plainly would like him to go with the Salvadoran right. Fortunately, others would not. Jeane Kirlpatrick, ambassador-designate to the United Nations, reiterated the other day that the most appropriate policy was to support the junta. She is right. In practical terms, that means two things. 0tAbove everything else, the violence must be controlled. That requires the new administration to do more than follow its natural inclination to help the junta resist violence on the left. It must also grant that the Carter administration has had good reason to encourage the junta to check the anti-popular violence on the right. Then, the new administration must make an unequivocal commitment to the agrarian reform. This program deserves to be seen not as some wold-eyed socialist scheme of the left, but as an effort to set up something like the family farms of the American Midwest -- to create a middle-class constituency.

Backing the junta is a long shot -- it has always been a long shot. It's simply the best bet available.