A COLONEL named Sigifredo Ochoa Perez has done the one thing best suited to demolishing the reform effort in El Salvador and ensuring a speedy guerrilla victory. He has rebelled against the government's authority, specifically, he says--making a distinction that in the circumstances is meaningless--against the person of the defense minister, Jos,e Guillermo Garcia. If he succeeds, Col. Ochoa will have transformed El Salvador's government from a struggling enterprise worth trying to influence and guide to just another roughneck regime in a place where the United States can have no further good reason to hang on.

The military largely served as the landlords' gendarme in El Salvador until 1979, when the officer corps made a historic break and set out on a reform path. Not every officer went along, but the effort was serious enough to reassure many of the military's old adversaries and to make revolutionaries on the far left fear their thunder would be stolen-- that's why they took up arms. A number of the old- guard officers quit or were forced out. Col. Ochoa, otherwise known for his professionalism, appears to have links with them, especially with the cashiered former major, Roberto D'Aubuisson, who is now president of El Salvador's constituent assembly. His words and his choice of associates suggest he would repudiate the reform course whose chief military patron since 1979 has been Gen. Garcia, the defense minister, and instead follow the retrogressive D'Aubuisson line.

Two years ago, it was widely suspected that Ronald Reagan would be a pushover for any Salvadoran colonel who would come along spouting right-wing anti-communism. But Mr. Reagan has surprised doubters by the extent to which he has stuck to the reformist democratic path laid out by his predecessor. Both have appealed to the American public to remain engaged in El Salvador on grounds that something reasonably centrist and democratic can eventually emerge. A guerrilla victory would wipe out that prospect. So would a successful defiance of government authority by Col. Ochoa. The moment that Americans get the idea that El Salvador is merely a place where colonels play games is the right time for the United States to end its aid.