With time running out, President Reagan's top foreign policy advisers struggled yesterday to find a formula that would avert almost certain rejection in Congress of the administration's long efforts to resume aid to the rebels opposing the leftist government of Nicaragua.

"Our hope now is the Senate Democrats," said one White House official, who said Reagan would meet today with Senate Minority Leader Robert C. Byrd (D-W.Va.) and other Senate leaders in a final bid to find a bipartisan compromise that would provide $14 million in "humanitarian aid" for the rebels, known as "contras."

"We don't have much of a fallback position because we've already fallen back considerably," the official said.

Because of what both congressional and administration sources said were problems of timing, a meeting between Reagan and Senate leaders scheduled for yesterday was postponed until today.

Byrd met for three hours in the Capitol yesterday afternoon with 10 other Senate Democrats, including John Kerry (Mass.) and Tom Harkin (Iowa), who described a three-page peace proposal given to them by Nicaraguan President Daniel Ortega during talks in Managua over the weekend.

White House deputy press secretary Robert Sims said the proposal, which had not been made formally to the U.S. Embassy, contained "nothing new" and did not provide for "a dialogue of reconciliation." He said its main purpose appeared to be aimed at influencing a vote against the aid proposal, which is scheduled to come up for a vote in both chambers on Tuesday.

Secretary of State George P. Shultz, leaving a White House meeting on the aid issue, was asked his opinion of the Ortega plan and flashed a thumbs-down signal.

The meeting chaired by Byrd was the third among key Senate Democrats in the last four days in an effort to find common ground on the issue of aiding the contras. According to a source close to the discussions, Byrd told Reagan after the first meeting Thursday that if the White House could reach a compromise with Senate leaders before the Tuesday vote the minority leader would find an "alternative legislative and procedural approach to the contra aid issue."

Administration sources, aware that some compromise in the Senate is their only hope for sustaining Reagan's hard-fought effort to provide at least a semblance of aid for the rebels, said they were willing to compromise on all procedural issues. These sources said that the administration's remaining goal was to provide the $14 million in aid, which could be used for food but not for arms, until the next fiscal year, and that Congress could determine the mechanism provided that it is an official agency of the government.

One source suggested that the likely mechanism would be not the Central Intelligence Agency but "an interagency group" that would be subject to close review by Congress to see that the money was not funneled indirectly into military aid.

A Democratic source said that, during yesterday's meeting chaired by Byrd, "one senator who has generally supported aid to the contras made a proposal, the general consensus of which the group was able to agree on. Whether they can agree on the particulars remains to be seen."

Reagan is focusing on the Senate because administration officials privately concede that they have almost no chance of winning an acceptable version of the aid request in the House.

They expect passage instead of a Democratic alternative that would provide $10 million for Nicaraguan refugees distributed by the International Red Cross or the United Nations and $4 million to Mexico, Colombia, Panama and Venezuela to administer any peace plan these countries -- known as the Contadora group -- might be able to produce.

In his Saturday radio speech, Reagan termed this plan a "shameful surrender" to the Sandinista government of Nicaragua. But administration officials said that, if the Senate passes a plan acceptable to Reagan, it may be possible to work out a compromise in a conference committee between the two chambers.

Appearing on NBC's "Meet the Press," Senate Majority Leader Robert J. Dole (R-Kan.) expressed some optimism, saying, "we think we can resolve this issue on Tuesday with pretty broad bipartisan support."

Instead of conferring with the Democrats yesterday, Reagan's leading policy advisers met among themselves. Shultz, national security affairs adviser Robert C. McFarlane, Defense Secretary Caspar W. Weinberger and CIA Director William J. Casey convened at the White House to assess prospects for a compromise.

The administration brushed off the Kerry and Harkin report that Ortega had offered a new proposal that would call for an immediate cease-fire, restore freedom of the press and make other conciliatory gestures if the United States halts support for the rebels.

Kerry said Ortega's offer contained "approximately six new elements" and provided "a wonderful opening" to resolve the conflict "without having to militarize the region." He and Harkin outlined the plan in a three-page memo, which was made available to the administration.

According to the memo, Ortega called upon the United States to discontinue direct and indirect support to the rebels and to enter immediately into new conversations with Nicaragua. He went on to guarantee access to these talks to congressional observers and to solicit U.N. and Red Cross assistance for the resettlement and repatriation of any citizen who wishes to live in Nicaragua or any neighboring country.

The memo said that Ortega pledged to "guarantee full freedom of the press and reaffirm political pluralism and fundamental freedoms" as well as "unconditional amnesty for any member of the contras who surrenders his weapons to representatives of the governments of Nicaragua, Honduras or Costa Rica."

A State Department spokesman said last night of the Ortega proposal: "We see this as mainly a restatement of old positions. There appear to be only two new points -- the conditional promise of cease-fire and the restructuring of the composition of the bilateral talks."

Including congressional participants in the talks does not appear to be workable, he said, because "they're dictating who will speak for the United States, or attempting to."

Former secretary of state Henry A. Kissinger, appearing on CBS' "Face the Nation," criticized Kerry and Harkin for sidestepping normal diplomatic channels.

"If the Nicaraguans want to make an offer, they ought to make it in diplomatic channels," Kissinger said. "We can't be negotiating with our own congressmen and Nicaragua simultaneously."