Administration strategists yesterday were promoting a plan that links congressional approval of military aid to the Nicaraguan contras with pledges of a last-ditch U.S. diplomatic effort to bring the Sandinista government and the contras to the bargaining table.

White House and congressional sources said the administration favored a proposal they described as a "nonlegislative compromise" calling for President Reagan to send a concessionary letter to Congress before next week's votes on the administration proposal to provide the rebels with $100 million in aid, including $70 million in military assistance.

The president would promise in this letter that he would not send military aid to the rebels for a specified period of time -- probably 60 days or 75 days -- while U.S. and Latin American diplomats tried to bring the Sandinista government to the bargaining table with the counterrevolutionaries, or contras.

El Salvador's president, Jose Napoleon Duarte, said last week he would enter into discussions with leftist guerrillas in his country if Nicaraguan President Daniel Ortega would negotiate with the contras. Reagan this week is sending Philip C. Habib, his new special envoy to Central America, to meet with Duarte and other leaders in the region.

Reagan said yesterday that the "tide is turning" on Capitol Hill in favor of his aid proposal, but Senate Majority Leader Robert J. Dole (R-Kan.) emerged from a meeting with Habib and said it will be "very, very difficult" to win Senate passage of the aid package. Dole said he was "always willing to compromise" and suggested that a proposal by Sen. Jim Sasser (D-Tenn.) to suspend delivery of the military aid for six months pending an attempt at a diplomatic solution "may be the basis" for a solution.

Reagan has said repeatedly that six months is too long to wait. One official said "the Sandinistas won't leave much of the contras as a fighting force" if the rebels receive no military assistance during the next half year.

Despite Reagan's claim that sentiment was swinging to his side on the aid package, administration officials and supportive Republican congressmen acknowledged that opponents of the proposal are presently ahead in the House. But sources on both sides said it was impossible to make a precise count because there are swing votes that could be decided by the wording of a Reagan letter or the amount of time the administration is willing to postpone military aid pending a diplomatic solution.

Under the proposal now being discussed by administration strategists and congressional leaders, that aid could be postponed indefinitely if the Sandinistas actually entered into negotiations with the contras.

But there was skepticism among some swing voters on Capitol Hill whether a presidential letter would be sufficient to gain approval of the aid package.

"It's got to be a lot more than a letter," said Rep. Dave McCurdy (D-Okla.), considered a key vote. "I've been the recipient of letters before."

Publicly, Reagan shifted gears yesterday toward a more bipartisan approach after a week in which White House communications director Patrick J. Buchanan pointedly attacked the Democrats for their opposition to the package and said the choice was "between Ronald Reagan and the resistance -- or Daniel Ortega and the communists."

Yesterday, Reagan told a group of supporters in the White House that "on this issue we must act not as Republicans . . . or Democrats, but as Americans." He went on to quote the late Sen. Henry M. Jackson (D-Wash.) as saying that "in matters of national security, the best politics is no politics," and to claim that an "anti-communist coalition" representing a bipartisan grouping on foreign policy was reemerging on Capitol Hill.

Administration strategists said that Reagan will push this bipartisan approach in private talks with congressmen all week, culminating in a nationally televised speech Sunday night.

The signs from the Senate did not appear favorable to the administration after a day in which Habib met with several Senate leaders. One Senate source said there is currently not enough support in the Foreign Relations Committee to approve the package and that it will probably be discharged to the Senate floor without a recommendation on Thursday.

Sasser told reporters that there is "better than a 50-50 chance" that the administration plan would be defeated in the Senate and said this would damage the credibility of Reagan's foreign policy.

Dole nonetheless scheduled debate on the proposal for next Monday, two days ahead of the March 19 House vote. This could mean that the Senate will vote first, in an apparent attempt to put more pressure on the House.

In the House the administration ran into difficulties on both sides of the issue. Rep. Michael D. Barnes (D-Md.) said the language of the president's request for the $70 million in military aid could free the Central Intelligence Agency to dip into additional contingency funds thought to amount to $400 million. Officials within the administration and on the Senate Select Committee on Intelligence disputed this interpretation.

Conservative Republicans, meanwhile, warned the White House that the administration would lose votes in its own ranks if Reagan compromised too much on military aid.