In Nicaragua (if nowhere else), the Reagan Doctrine is dead. Whatever replaces it will be crafted by Congress. In a display of lame-duck ineptitude that makes its handling of Supreme Court nominations look deft, the Reagan administration has allowed the conduct of U.S. policy in Central America to pass into congressional custody and, more specifically, into the hands of House Speaker Jim Wright.

That is the real message in Secretary of State George Shultz's announcement last week to the Organization of American States that the administration will put off until next year its once high-pitched plea for an 18-month, $270 million extension of military aid for the Nicaraguan resistance forces. That's also what's behind the largely phony fuss that the White House is making over Wright's "intrusion" in the "Guatemalan" peace process. Wright was invited in -- by the president. By the administration's own default, he has emerged as the man in the U.S. power structure whom the Central American peace planners most trust and respect.

Some would argue that last August's disingenuous effort to rope the speaker into a ''bipartisan'' Wright-Reagan peace plan marked the beginning of the end of the administration's relevance. You could claim that the end was nigh in Congress a month or more ago, at least in the minds of those who (a) took the speaker's word for it or (b) could count -- which the White House neither did nor could.

No matter. The end came last week for all of Shultz's tough talk: ''This does not mean we will sit idly on the sidelines if the Sandinistas try to strike for a military victory. We will not abandon the resistance fighters . . . with their weapons exhausted. We will not permit the peace process to become a shield for the physical elimination of the Nicaraguan resistance.''

Reading those words you might actually believe that the administration has choices. When Shultz goes on to say, ''We will give peace every chance,'' you might believe, as well, that the United States won't be providing military aid to the contras for the rest of the year and beyond.

But then, if you had taken the administration at its word at any of several points along the way in the past six years, you would have believed that the original contra aid was designed only to interdict Nicaraguan support for rebels in El Salvador; that the president really thought he could make the Sandinistas say ''uncle,'' abandon Marx and Lenin and take up democracy; that the administration was actually serious about the Wright-Reagan plan and not simply trying to show up the Sandinistas, at Wright's expense, as spoilers, by way of making the case for continuing military aid.

You might also have believed Ronald Reagan when he told the OAS on Oct. 7 that he would ''fight for'' the $270 million military aid package ''as long as there's breath in this body,'' or Shultz when he told the House Foreign Affairs Committee on Oct. 13 that he would put that aid package on Congress' platter before Thanksgiving Day. You might even have taken seriously the administration's fallback proposal for $30 million in nonlethal aid, which has now been shelved along with the $270 million arms request.

What happened? The answer is simply that the administration's performance over the years has made disbelievers of a majority in both houses of Congress and, most intensely, of Speaker Wright and the Democratic leadership in the House. In the end, it took a face-to-face confrontation between Wright and White House officials to convince the administration that all it would get out of a fight would be a smashing defeat that would only serve to shatter contra morale.

When the administration tried to threaten the Democrats with the political implications of ''abandoning'' the ''freedom fighters'' in the field, Wright knew better. He and his cohorts knew what the administration also knew but didn't want to say out loud: there is enough military aid in the pipeline from last year's $100 million appropriation to keep the contras going at least through next January and perhaps longer.

What then? Nobody knows. It will depend on how Congress, in an election year, reads the progress of the Guatemalan peace plan agreed to by Nicaragua, together with El Salvador, Honduras, Guatemala and Costa Rica, whose Nobel Prize-winning president, Oscar Arias, took the lead. It will depend on events -- how the Sandinistas play (or misplay) it, how their neighbors react, Moscow's role behind the scenes -- that are beyond even Congress' capacity (never mind the administration's) to control.

Whatever happens, last week's capitulation by the administration makes it clear that, as far as U.S. policy is concerned, it won't be the administration's call. Congress will be taking the measure that matters of how much of a chance the peace process has earned by January. The test that will be applied will be a lot more realistic and reasonable than the test the Reagan administration would apply