President Reagan told Nicaraguan rebel leaders yesterday that they are "the future of Central America" and promised to go back to Congress "as many times as it takes to win" approval of military assistance for their contra movement.

In a defiant speech in the East Room a day after the House rejected a $100 million aid package for the anti-Sandinista rebels, Reagan said his administration would "spare no effort and give no ground in supporting the democratic resistance in Nicaragua."

With a Senate vote on the aid package probable on Wednesday, the president's message was that aid is urgently needed because the Sandinistas have been "emboldened" by the House action to move militarily against the rebels along the Honduran border.

"Time is of the essence," the president told 200 cheering supporters. "Every day that passes, the freedom fighters of Nicaragua are left to face Soviet helicopter gunships with hand-held rifles."

White House chief of staff Donald T. Regan and spokesman Larry Speakes, who last week predicted a victory for the aid package in the House, expressed optimism yesterday about its chances in the Republican-controlled Senate. This time, however, there was some Democratic agreement, with Senate Minority Leader Robert C. Byrd (D-W.Va.) predicting that Reagan "would probably get most of what he wants in the Senate."

The administration package will contain a concession delaying three-fourths of the aid for about 90 days, the same offer Reagan made to the House in an unsuccessful attempt to sway swing voters. However, it would allow defensive weapons such as antiaircraft missiles to be supplied to the contras immediately, a proposal that could prove controversial.

Sen. Jim Sasser (D-Tenn.) called the delay, which would have been included in an executive order if the bill had passed the House, a "fig leaf compromise" that simply delays the aid and includes "conditions that will lead to a failure."

However, Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) said these provisions would give Reagan a broader base of support.

Senate Democrats began work on several alternatives to the Reagan plan, and one White House official expressed concern that "we could be killed by a thousand amendments."

In assessing the reasons for their setback Thursday and mapping next week's strategy in the Senate, White House officials said they would not repeat the partisan confrontation of the House debate.

"We will not go back to the op-ed phase," said a senior White House official, referring to a March 5 article in The Washington Post by White House communications director Patrick J. Buchanan. The article said that in its vote on contra aid "the Democratic Party will reveal whether it stands with Ronald Reagan and the resistance -- or Nicaraguan President Daniel Ortega and the communists."

White House officials contend that Buchanan's article helped focus the debate but disagree about whether it was productive in terms of votes. They agree, however, that a repetition would backfire in the Senate, where Sen. Nancy Landon Kassebaum (R-Kan.), a key swing vote, said that Buchanan's statements "demean our democratic institutions and demean the presidency."

One White House strategist said yesterday that the administration would use "a refined hammer" instead of a "sledgehammer" and "leave the sledge in the barn."

Reagan and his audience were in an emotional mood yesterday as they gathered in the East Room to recommit themselves to supporting the contras.

The president began the session by holding aloft a button that said, "If you like Cuba, you'll love Nicaragua." He ended it by calling on three leaders of the Nicaraguan rebels -- Adolfo Calero, Alfonso Robelo and Arturo Cruz -- to come and stand with him.

"I want to tell you something," Reagan said. "We're in this together. The future of Central America is with democracy and all those who are fighting for freedom. You are the future of Central America."

In response, Calero held up a clenched fist and shouted, "Viva Reagan," a chant taken up by the audience after the president finished his remarks.

In vowing that he would "never give up" his efforts to obtain military aid for the rebels, the president said "the cause of freedom and hope and democracy suffered a temporary setback yesterday in the House . . . . I underline temporary. History will record yesterday's vote as wrong, tragically wrong."

Administration officials said that, beginning with his weekly radio speech today, the president will stress the theme that the contras will be unable to survive militarily if the aid is delayed. In private meetings with House members last week, Oliver North, a senior member of the National Security Council staff and liaison to the contras, argued that rejection of the package would distress Honduras, which provides base camps for the rebel forces.

Yesterday, according to a senior White House official, U.S. diplomats privately reassured Honduran government officials that the aid was likely to be approved by the House next month.

Rep. Michael D. Barnes (D-Md.), an outspoken foe of the aid package, said yesterday that the administration should use the "brief window of opportunity" provided by the House action to renew diplomatic efforts.

But administration officials maintained there was little the United States could do diplomatically without the aid package, which comprises $70 million in military aid and $30 million in humanitarian assistance. Reagan's view is that the Sandinistas will negotiate only if faced with military pressure.

As if to underscore this view, the president's special envoy to Central America, Philip C. Habib, was in California on personal business. He is not scheduled to return until after Easter.