With less than a month to go before the Nov. 7 deadline for implementing the five-nation Central American peace agreement, President Reagan appears to be losing the decisive battle of his six-year campaign to combat Nicaragua's Marxist government through the proxy use of U.S.-supported contra guerrillas.

Even senior Reagan administration officials who have backed the contras for years admit that the contra policy -- linchpin of the Reagan doctrine's determination to thwart communism worldwide through wars of national liberation -- is in serious danger of being discarded by a Congress that thinks it sees an alternative to the president's pursuit of military solutions in Central America.

That is likely to become newly evident today when Secretary of State George P. Shultz gives the House Foreign Affairs Committee an "update" on the Central America situation. Shultz, echoing the line taken by the administration since the peace agreement was signed last Aug. 7, is expected to argue that Congress should give the contras $270 million in new military aid as insurance to force Nicaragua's Sandinista leaders to carry out the accord's call for democratization and a halt to subversion in the region.

In past years, this argument was a successful part of Reagan's strategy of making Congress vote "again and again" on contra aid until shifting political considerations and respect for the president's popularity chipped away the opposition.

However, as Shultz has learned in similar meetings since last August, the dominant tendency in Congress right now is to turn a deaf ear to Reagan's insistence, reiterated in a speech last week to the Organization of American States, that contra aid is "a moral obligation" and "the essential guarantee" of Sandinista good conduct.

Instead, the legislators are responding sympathetically to the arguments of Costa Rican President Oscar Arias, principal author of the peace plan, and of House Speaker Jim Wright (D-Tex.), who has become its foremost champion on Capitol Hill, that to talk about contra aid at this time is counter to the spirit of Arias' plea to "give peace a chance."

In short, the administration has found that it can no longer pursue a strategy of putting Congress on the defensive, even when it is controlled by the Democrats, by arguing that voting against contra aid would be seen by the public as voting to do nothing about the Nicaragua problem. The argument that contra aid was the only game in town faded Aug. 7 when the presidents of Costa Rica, Nicaragua, Guatemala, El Salvador and Honduras gathered in Guatemala to sign the peace accord.

The predicament in which the administration now finds itself began to take shape on Oct. 5, 1986, when a private American transport plane was shot down in Nicaragua, triggering suspicion that the administration was attempting to circumvent congressional restrictions on aid to the contras. Soon afterward came the Iran-contra affair, with its months-long series of revelations about the unauthorized diversion of millions of dollars to the contras.

By late summer, when the nationally televised Iran-contra hearings had run their course, it was clear that the administration had lost its ability to intimidate Congress by making contra aid a high-profile political issue for swing-vote members from conservative, mostly southern, districts. Then in August came the chain of events that created an alternative toward which the emerging anti-contra aid sentiment could gravitate.

It began when White House chief of staff Howard H. Baker Jr. approached Wright with the idea of seeking a bipartisan approach to Central America. Some Democratic members of Congress still question whether Baker was genuinely sincere about finding a new approach or was trying to snare Wright into supporting renewed contra aid. In any case, his approach gave Wright, who had been pondering the issue for a year, an opportunity to transcend the bounds of his official role as a legislative leader and make himself into an unofficial secretary of state for Central American affairs.

With Wright's encouragement, the bipartisan initiative that he and Reagan announced Aug. 5 was used by Arias as leverage to prod Nicaragua into joining its four democratically ruled neighbors in the peace agreement they signed two days later.

Reagan and his senior advisers, bound to the contra policy by their distaste for the Sandinista regime in Managua, reacted to the Central American plan with a confusion that tilted increasingly toward hostility. But Wright moved into the void and became the broker who blessed the agreement. He encouraged the Central Americans to go forward with implementation despite the administration's reservations, and pointed the way for Congress to push contra aid into the background with the argument that the peace process first should be given a chance to succeed.

While that was happening, the administration has shown an inability to get a grip on the situation and put its imprint on the unfolding process. Instead, it consistently has sent mixed signals, teetering between half-hearted lip service to the Central American plan and attempts to assuage the Republican far right by denouncing the Sandinistas as untrustworthy communists and vowing never to desert the contras.

The administration has made no secret of its belief the Arias plan lacks many of the safeguards for U.S. interests that were built into the short-lived Wright-Reagan initiative. In particular, it cites the Central American agreement's failure to specify that Nicaragua stop receiving military aid from Cuba and the Soviet Union, its silence about whether the Sandinistas must negotiate with the contras and its failure to define with precision how civil wars in the various countries are to be resolved through cease-fires and amnesties for rebels.

But these concerns have been obscured by the administration's shrilly ideological tone and by the desire of many members of Congress and the Central American governments to put the Nicaragua issue aside even if the solution is temporary.

Those who support the Arias plan say it sounds as though the administration, finding that it cannot oust the Sandinistas through military force, now is demanding that the Managua government assent to a negotiated surrender of its power and accept a U.S.-dictated model of democracy for Nicaragua.

That, backers of the plan in Congress and in Central America say, is a totally unrealistic expectation for an agreement whose aim is not to dictate the terms of Nicaraguan sovereignty but to ease regional tensions and permit a limited liberalization inside Nicaragua that could provide a basis for greater democratization in the future.

It is not an ideal solution, its Democratic supporters acknowledge. But, they add, the moves made by Nicaragua so far toward implementing its provisions have achieved more in terms of easing the repression there than anything accomplished by the contras in six years of guerrilla warfare. And their companion argument that contra aid should be kept off the table until the agreement has been given a fair trial clearly seems increasingly likely to prevail over the administration's frustrated appeals for quick action on its request for $270 million in aid.