Not just leading congressional critics of the administration's Central American policy but even some within the White House are coming to the same uncomfortable conclusion: some combination of diplomatic, economic and political euthanasia is the only realistic way out for the Nicaraguan contra cause. More and more, the question is coming down to what would be the most painless way to pull the plug.

That's not yet the focus of public debate. Even as regional peacemakers prepare for the Guatemala accords to go into effect on Nov. 5, Secretary of State Shultz is doggedly tub-thumping for a $270 million, 18-month renewal of U.S. military and other aid to keep the heat on the Managua government to mend its Marxist-Leninist ways. The administration effort persists as if nothing out of the way is happening.

But plenty is happening -- and just about all of it works to stiffen congressional resistance. The outrage stirred by last summer's Iran-contra hearings will be rekindled by the final findings of the House and Senate select committees. The various compromises in the Guatemala peace plan are so fragile that two cosignatories, El Salvador and Honduras, have warned Congress to hold off on more military aid for the contras.

The Nobel Peace Prize awarded to Costa Rica's President Arias for his role in the peace effort has played to regional pride in a way that serves to strengthen innate resistance to ''North American'' interference. Finally, the stock market crash, for all its seeming irrelevance to geopolitics, has put the budget deficit crisis on center stage and U.S. overseas spending under particular pressure. ''That $270 million looks more than ever like big money,'' says one congressional aide.

The cumulative effect of all this is to make it much easier for those who vote down contra aid to duck predictable soft-on-Sandinistas, who-lost-Nicaragua punches in next year's political campaign. Already House Speaker Jim Wright has picked up on the pleadings of Arias (who also would have us hold off on further contra aid for the time being) to ''give peace a chance.''

Conceivably, Nicaragua's President Daniel Ortega could find a way to blow it. In the past, he has shown himself not too strong on political smarts. By continuing to quibble over whether he'll talk to exiled political leaders of the contra movement or only to field commanders, or by too blatantly making a sham of his concessions to ease internal repression, he could help make the administration's case and even disgust his partners in the Guatemala plan.

But that's not the expectation of congressional experts. They figure the Sandinistas will ration out reforms in sufficient quantities to keep the peace plan alive and promising. And that apparently is what the White House also figures -- and fears. ''We'll make our own determination about what . . . constitutes compliance with the Guatemala peace plan in terms of all the range of decisions we have to make in Central America,'' White House spokesman Marlin Fitzwater declared recently.

That's pure bluster. Granted, the Guatemalan formula falls well short of the administration's previously professed objectives. But Congress is unlikely to be impressed by the spectacle of the Reagan administration setting itself up as holier than Nicaragua's next-door neighbors about the security of their own region.

For the sake of tranquillity and an end to bloodshed and destabilizing danger from assorted insurrections in their own countries, El Salvador, Guatemala, Honduras and Costa Rica have significantly different priorities than those of the administration. They would demand less in terms of Nicaraguan internal reforms than Ronald Reagan's dream of delivering ''full democracy.''

If the price for peace includes rolling up the contra resistance movement in exchange for assurances that the Sandinistas will present no external threat to the area, they would pay it -- even if they won't say it. The same may be said of many in the mounting opposition in Congress to contra aid.

It is not a pretty prospect; nobody is suggesting that the repatriation or resettlement of the contras would be easy. But the alternative is open-ended U.S. aid to keep the contras killing and dying, in inconclusive pursuit of Ronald Reagan's U.S. interests. For us to do this, when the contras' chances of satisfying their own interests are so slim, is hardly prettier.