BY THE USUAL small margin, Congress has again changed its mind on the contras. First, in 1981, it said yes, then no, then yes and now no again, despite a ferocious closing charge by President Reagan. Congress has not shrunk from supporting other anticommunist insurgencies, but it has shown a deep ambivalence about this one. We understand why, having ourselves come only lately to contra aid -- on grounds that the Central American peace and democracy plan had created a bargaining context in which military pressure had its uses.

There will now be as clean a test as there ever is in these matters of whether the Sandinistas have been making political concessions in part because of the contras or, as a congressional majority evidently believes, strictly because of the urging of their fellow Latins. The argument in fact was that further military aid to the contras would deter the Sandinistas from fulfilling their democratic pledges, whereas denying such aid to the contras would spur the Sandinistas on. Now we shall see.

The administration is looking for other ways to keep the contras going. It is a quest burdened by the general alertness against the official end runs made when Congress halted aid back in 1984, and by the general fatigue. The contras stockpiled some supplies against an aid cutoff. But there can be little doubt that a militarily one-sided ''peace'' is coming in Nicaragua. The real question has always been whether reasonable, let alone irreversible, progress toward democracy is also coming. Congress has ensured the answer will emerge exclusively on the diplomatic track.

Speaker Jim Wright says Congress would reconsider contra aid if the Sandinistas were to ''misbehave in extreme ways.'' President Oscar Arias of Costa Rica, principal author of the peace plan, says that in such circumstances he would go to the Organization of American States and ask it to commission a military rescue mission by . . . the United States. Can anybody really imagine cranking the contras back into operation, or sending in the Marines? It's easier to hope things won't sour.

Whether Nicaragua is ''lost'' or ''won,'' there will be a hot debate about it. And why not? It's important and a proper subject for political argument. Meanwhile, the democrats of Nicaragua deserve everyone's urgent attention. The administration's appeals for them are sometimes regarded as suspect and dismissed as too-fervent anticommunism. Let Congress and other policy critics lead the band insisting that the Sandinistas make good on their promise of a political opening.