A long weekend spent watching the Contadora countries struggle to advance their Central American peace initiative leaves one respectful of the Latins' effort but convinced they cannot accomplish the mission being thrust upon them by many Americans who are troubled by the Reagan administration's policy.

That mission is, of course, to do something to justify a reasonably honest vote in the aid to the Nicaraguan contras. As the foreign ministers of the five (core) Central American states and of eight other Latin governments gathered here, the notion was widespread in Washington that Contadora is a necessary and feasible alternative to contra aid and that the Contadora 8 could, if they would only extend themselves more rigorously, do the job.

In this latest "last-ditch" stand, however, Contadora, though it did not entirely crash, fell painfully short. Its members went into an extra session yesterday but still failed to get Nicaragua's commitment to sign their laboriously drafted and amended regional peace treaty or "Act." They also lost their grip on two projects meant to demonstrate Contadora's capacity for the concrete: setting up monitors on the Nicaraguan-Costa Rican border and inducing some modest but specific Sandinista step toward internal Nicaraguan reconciliation.

The Sandinistas stonewalled, demanding once again a prior commitment by the United States to stop trying to overthrow them, and rejecting the Contadora logic that some conciliatory moves on their part could contribute to the very relaxation by the United States that they seek.

It did not seem to bother the Sandinistas that they were thwarting a process whose patrons see it as a boon first of all to Nicaragua, or that the Reagan administration could not fail to use their intransigence -- the right word -- to make its case for contra aid. For whatever reasons -- they are communists, they are scared, they are smart, they are stupid -- the Sandinistas stood their ground.

So let us all stop pretending that Contadora is the easy way out. Jeane Kirkpatrick was right on the mark when she said (op-ed, March 31) in a column that had stung many of the delegates here that Contadora is now "less a diplomatic process than an incantation for a good many opponents of assistance to the contras."

The key question is why is this so. Our former United Nations ambassador emphasizes the assorted deficiencies of the Latins, not least Nicaragua's own contempt for Contadora. She ignores the factor central to the Latins' own diagnosis: the overwhelming weight of the United States, which is conducting another policy.

American policy is interventionist, directed finally at ending Sandinista rule. No doubt in their hearts many (most?) Latins sympathize with this goal. They can see Nicaragua becoming a second Cuba, and it alarms them.

But their tradition, their social fragility and their politics keep them from embracing the American goal. Their actual policy is directed simply at containing Sandinista expansion: furthering pluralism would be a bonus, one best sought, they believe, by calming things down.

Contadora, in short, serves the Latin purpose of live and let live -- living even, yes, with a Nicaraguan regime that may become still more openly communist. If pursued -- and remember that American power is preventing it from being pursued -- Contadora could not really be counted on to serve the American purpose of reversing a communist power play and bringing democracy to Managua. It is better not to have illusions about Contadora.

It is also better not to have illusions about the contras. To imagine that they are or can feasibly become a fit instrument of the administration's ambitious policy goals defies almost everything one hears about their political and military frailties. To paraphrase Jeane Kirkpatrick, the resistance is less a military option than an incantation for a good many partisans of assistance to the contras.

But the unhappy truth is that poor Central American has been caught up in a dead-serious great-power proxy confrontation. First Jimmy Carter, then Ronald Reagan saw the communist side gain major advantage. It is now very late in the day and Reagan is frantically playing catch-up. But neither the contra option he is pressing nor the Contadora option the Latins submit in its place is up to the heavy demands being put on it.

It is a dilemma that cries for a more subtle treatment than the American political system has so far given it.