With negotiations to get Soviet troops out of Afghanistan nearing what most observers describe as a final "end game" phase in Geneva, the Reagan administration may find that its 1985 pledge to "guarantee" a settlement will impose painful policy choices.

The choices may boil down to these: continuing U.S. support for the anti-Marxist Afghan resistance, which could jeopardize a final settlement, or cutting off crucial aid to the mujaheddin guerrillas and trusting Soviet assurances that it will indeed quickly withdraw all its 115,000 troops.

The stakes are tantalizing: the administration suddenly sees the opportunity of making possible the first Soviet withdrawal from a Third World country in modern times.

A Soviet pullout from Afghanistan would constitute a major victory for the so-called Reagan Doctrine aimed at rolling back Soviet gains in the Third World since the mid-1970s. President Reagan has long portrayed himself as the West's foremost champion of anticommunist "freedom fighters" around the world but has no victories yet to show for his commitment.

But congressional conservatives and other supporters of the Afghan resistance are now concerned that his administration is about to "sell out" the rebels' cause in its haste to see a Soviet withdrawal accomplished before Reagan leaves office next January.

The immediate focus of these worries is a December 1985 administration commitment to become a "guarantor" of peace accords being negotiated for the past six years under U.N. auspices in Geneva to end the war in Afghanistan.

The accords, which may be signed in March, make no provision for a political settlement or a role in a new government for the U.S.-armed Afghan resistance, which has declined to participate in the talks between Afghanistan and Pakistan. The accords commit the United States to end all military aid to the resistance at the start of a Soviet pullout -- before there is any certainty of its completion.

Soviet leader Mikhail Gorbachev offered this month to start a withdrawal May 15 and complete it in 10 months provided the peace accords are signed by March 15 and the United States cuts off its massive military assistance -- $600 million last year -- to the resistance. His announcement has convinced skeptics in the U.S. intelligence community that Gorbachev is now serious about a Soviet pullout from Afghanistan this year.

But resistance supporters here are concerned that the administration will leave the guerrillas without sufficient military or political support to complete their eight-year-long bloody struggle to overthrow the communist-dominated Kabul government.

The main conservative concern now focuses on the U.S. commitment as a "guarantor" of the U.N. peace accords, made in December 1985 after the first Reagan-Gorbachev summit to act as a "guarantor" of the U.N. peace accords. Precisely what this means remains unclear, partly because the exact wording of the accords has yet to be made public.

Sen. Gordon J. Humphrey (R-N.H.) and Rep. Charles Wilson (D-Tex.) charge that the State Department signed "a secret agreement" in 1985 unbeknownst to Reagan, committing the United States to serve as an accords "guarantor." They are asking that the accords be made public and the nature of the U.S. commitment be made clear.

Twenty-seven senators and 49 congressmen have written Reagan expressing their concern as well.

Afghan resistance leaders have never partipcated in the U.N.-sponsored peace negotiations between Pakistan and Afghanistan in Geneva and oppose the agreement now in the making, largely because they have been left out.

Efforts have yet to bear fruit by U.N. negotiator Diego Cordovez in behind-the-scenes talks that began quietly last year to shepherd agreement on the formation of an interim coalition government that would simultaneously set the scene for a political settlement and bring the Afghan resistance into the process.

The administration, which once pressed Gorbachev to announce a speedy withdrawal timetable, now finds itself in the embarrassing position of having one in hand while its key ally, Pakistan, is balking at signing the accords before a new interim government including the Afghan resistance is established.

One of the four "instruments" making up the accords to be signed by the United States and the Soviet Union calls for an end to all outside interference in Afghanistan's internal affairs. This is understood by all parties to mean an end to all military aid for the Afghan resistance in return for the Soviet withdrawal.

State Department lawyers have only recently begun studying precisely what U.S. obligations are as a "guarantor." However, they already say that the only thing the United States is "guaranteeing" is its own behavior regarding the non-interference provisions and that it has no legal responsibility for the entire agreement.

Whether this "narrow interpretation" of the term "guarantor" is acceptable to U.N. and Soviet officials is unclear.

A U.N. official said that Washington and Moscow as guarantors are expected to give political support to the accords even if they are not held legally responsible for the success of their implementation.

State Department officials say that the U.S. commitment, first announced by Deputy Assistant Secretary of State John C. Whitehead in December 1985, to act as a guarantor was always contingent on a satisfactory Soviet withdrawal timetable and other unspecified conditions.

A year ago, U.S. Ambassador to Pakistan Arnold L. Raphel told Humphrey during his Senate confirmation hearings that the administration's idea of an acceptable timetable was one based solely on logistical considerations. He did not specify how many months he had in mind.

Asked about the U.S. role as guarantor, Raphel told Humphrey that the United States had told Cordovez "we would lend our political support to the settlement only if it is fair and equitable and meets our criteria. We have not signed carte blanche that whatever is negotiated we are going to support it."

Conservatives blame Raphel, who was deputy assistant secretary of state for South Asian affairs when the commitment was made, for proposing the "guarantor" idea.

But State Department officials say Secretary of State George P. Shultz decided on the commitment to provide impetus to the slow-moving U.N. peace talks and to meet a key Soviet demand that U.S. aid to the Afghan rebels would halt if Soviet troops were withdrawn.

Reagan's National Security Council staff approved the decision, but whether he was aware of it at the time remains unclear. White House spokesman Marlin Fitzwater has insisted he was. But Fitzwater has had difficulty explaining why the president several times said that U.S. military aid to the Afghan resistance will continue until all Soviet troops are withdrawn.