The Senate last night narrowly approved President Reagan's modified request for $100 million in military and humanitarian assistance to the Nicaraguan rebels. The vote was 53 to 47.

Eleven Democrats and 42 Republicans joined in the winning vote, which came one week to the day after the House ignored last-minute changes proposed by the White House and rejected a similar aid request, 222 to 210. Eleven Republican senators voted against the administration request.

In a statement, Reagan said he was "deeply pleased" by the vote, which he said "is sure to send a profoundly reassuring signal to the freedom fighters in Nicaragua and to Nicaragua's threatened neighbors." Earlier in the day, at a political event in New Orleans, Reagan said the Sandinista offensive in Honduras "is a slap in the face to everyone [in the House] who voted against aid" to the counterrevolutionaries, or contras. [Details on Page A27]

The package approved yesterday essentially put into legislative form the same 11th-hour modifications the White House had proposed unsuccessfully on the eve of the House vote.

The White House and Senate Republicans wanted a wide margin so they could claim a bipartisan mandate and improve Reagan's chances in the next House vote, scheduled for mid-April.

"It was simply not to be on this issue," said Sen. Richard G. Lugar (R-Ind.), chairman of the Foreign Relations Committee, remarking afterwards, "It's better than 51 or 52."

Still, Lugar called the vote "a good strong statement," especially in light of last week's House vote, and said it would provide "a good basis for the House debate."

Sen. Jim Sasser (D-Tenn.), who sponsored the leading Democratic alternative, said the outcome was "certainly no mandate for the president's policy."

Before passing the White House plan, the Senate resoundingly defeated Sasser's alternative. It would have provided $30 million in humanitarian assistance only, mandated bilateral U.S. negotiations with the Nicaraguan government with no preconditions and required congressional approval for any additional funds.

Sasser billed his proposal as "the last train leaving the station" for those hoping for "a last effort at negotiations" instead of the White House plan, which he termed "a single-dimension military alternative."

"It's time to agree on where we're going in Central America before we find ourselves with United States troops on the battlefield and the body bags coming home once again," Sasser said.

Sen. Pete Wilson (R-Calif.), speaking against the Sasser amendment, termed it "naive," however. He said "people will die as a result of these naive good intentions" because the Nicaraguan government would use the time to stall and wait for a "less zealous" American president than Reagan.

The Sasser proposal, admittedly softened to attract the votes of liberal Democrats who might otherwise have voted against any assistance, was defeated 67 to 33, with Sen. Mark O. Hatfield (R-Ore.) joining 32 Democrats on the losing side. Fifteen Democrats voted against the Sasser plan.

Sen. Alan Cranston (D-Calif.) made one final attempt to place a Democratic imprint on the measure. He proposed that no military aid be given to the rebels if the Nicaraguan government agreed to a cease-fire, ended its current state of emergency and entered into "good faith" bilateral negotiations with the United States.

Senate GOP leaders moved, however, to table the Cranston plan and won, 66 to 34, after Lugar read excerpts from a letter from Reagan opposing such an amendment.

"Conditioning our aid to the Nicaraguan resistance on the initiation of direct bilateral talks, without first requiring that the Sandinistas talk to their own internal opposition, would seriously undercut our friends in the region and our foreign policy worldwide," the president wrote.

Shortly afterward, the Senate approved the White House proposal.

Under the plan, $25 million could be spent immediately for nonlethal assistance and so-called defensive weapons, which apparently would include portable surface-to-air missiles.

The remaining funds could be spent 90 days later in installments of $15 million every three months. The funding would start July 1 if the president determined that there was no reasonable chance to negotiate major changes in Nicaraguan policy sought by the United States.

Efforts would be made, the legislation provides, to reform the Nicaraguan rebel movement and organize a better opposition to the leftist Sandinista government, including making the military arm more responsible to civilian leadership.

Congress could disapprove the further expenditures, but only by a two-thirds vote. The plan provides for bilateral U.S.-Nicaraguan negotiations only if the Managua government begins negotiations with the contras -- something Managua has said repeatedly it will not do.

Earlier in the day, the Senate rejected two alternatives that generally were poles apart in the argument over contra aid, a major foreign policy issue of this election-year congressional session.

Senators first rejected, 74 to 24, an amendment by Sen. Edward M. Kennedy (D-Mass.) that would have cut off all military assistance. Five Republicans joined 19 Democrats in voting for that proposal, which received seven more votes than a similar proposal did last year.

Then the Senate voted down, 60 to 39, a proposal by Sen. Jesse Helms (R-N.C.) that would have given the Nicaraguan government until May 15 to stop exporting weapons and revolution, expel all Soviet and Cuban personnel and make certain democratic reforms. Otherwise, the White House would be allowed to spend all $100 million, regardless of other criteria.

The only amendment adopted on the floor was sponsored by Sen. Alan J. Dixon (D-Ill.). It would prohibit use of U.S. trainers or advisers inside Nicaragua, something White House officials have repeatedly said the administration would not do.

For several days, Democratic and Republican leaders had worked on a possible bipartisan compromise, but those efforts broke down Wednesday with each side accusing the other of being intransigent.

Lugar acknowledged that the president's proposal may have lost some votes because of the inability to find common ground on the issue of bilateral negotiations.

The White House did, however, make sufficient changes to win the votes of four senators who had expressed concern about the composition of the Nicaraguan opposition and possible human rights violations.

Those four were Republicans Nancy Landon Kassebaum of Kansas, William S. Cohen of Maine and Warren B. Rudman of New Hampshire, and Democrat Sam Nunn of Georgia.