Senate Republicans, bolstered by administration reports of a Nicaraguan incursion into Honduras, moved closer yesterday to winning enough swing votes to claim a bipartisan mandate for President Reagan's request for $100 million in U.S. aid to the Nicaraguan insurgents, or contras.

Key GOP aides said late yesterday that the nearly finished version of a compromise proposal includes most of the provisions favored by the White House. The package is expected to be unveiled today as the Senate begins 10 hours of scheduled debate on the aid request.

Sen. Jim Sasser (Tenn.), the principal Democratic negotiator in the talks, agreed early in the afternoon that "some progress" was being made toward an agreement, but that there was still "a ways to go" before a large number of moderates would support the measure.

Sasser predicted, however, that administration reports of the Nicaraguan incursion could have "a very chilling effect" on any effort to limit aid to the contras. That seemed to be the case as the end of the day neared.

In both Senate and House, Democrats opposing contra aid and those advocating tighter congressional controls over any such assistance expressed anger with the reported invasion.

Some Democrats condemned the attack while deploring an escalation of military activity.

"This escalation is the predictable result of policies that seek to deal with Central America's problems with war rather than through diplomacy," said Rep. Michael D. Barnes (D-Md.). "Further escalation is almost inevitable if we don't change our policies."

Others sharply criticized Nicaragua's action as self-defeating.

House Speaker Thomas P. O'Neill Jr. (D-Mass.), who throughout the House debate warned of eventual U.S. involvement if aid were sent to the contras, said he would still vote against any aid request, but termed the reported Sandinista invasion a "tremendous blunder."

Last year, many in the House took considerable heat for helping reject a White House request for military aid for the contras and then were stung when, a few weeks later, Nicaraguan President Daniel Ortega visited Moscow.

This year, even more fierce anticommunist rhetoric was used against those opposing the Reagan plan, and some legislators who support modifications to the proposal or who oppose it outright said Ortega's apparent action again had played into the hands of his harshest critics.

"If it's true, he's one-upped his own trip to Moscow," Sen. Sam Nunn (D-Ga.) said.

Rep. Thomas S. Foley (D-Wash.), the House majority whip, termed the action "another example of the stupidity" of the Sandinistas.

"Nicaraguans have a habit of wishing themselves the most hostile" reaction in the United States, Foley said. "If they're determined to create the role of international outlaws here, they're probably going to be satisfied."

Some senators were initially doubtful of the reports of a Sandinista incursion. "The timing of this is not purely conicidental," Sen. Paul Simon (D-Ill.) said.

Sen. Christopher J. Dodd (D-Conn.) said the report "ought to be thoroughly explored before we buy it as an accepted fact." Dodd indicated he was somewhat skeptical of the White House presenting the reported incursion as "a perfect example of why we must have these contras."

Senate Republican leaders had been hoping to fashion a compromise that could win 55 or 60 votes to strengthen Reagan's hand for the next vote on contra aid in the House, which last week defeated the administration's aid request by 12 votes. The House is expected to take up contra aid again when it returns from Easter vacation.

After the House defeat March 20, Reagan ordered Senate GOP leaders to put into legislative language several modifications he had told the House he would make through executive order.

Some House moderates said at the time they would have supported the aid package if the modifications were in the form of law.

There were some major points of disagreement between the White House and leading Senate moderates. The senators favored bilateral negotiations between the United States and Nicaragua's Sandinista government. The White House had opposed such talks, saying they would nullify the legitimacy of the contras.

Yesterday, however, negotiators seemed headed toward an agreement that would encourage such talks but not necessarily mandate them. Further, it would not allow the Sandinistas to use the lack of talks as an obstacle to diplomatic efforts, aides said.

The proposed compromise also would delay the spending of most of the funds for 90 days, but would allow purchase during that time of so-called defensive weapons. After that time, if certain criteria had been met, the administration could spend the remainder of the funds -- unless Congress passed a measure of disapproval.

At the same time, the administration would have to make efforts to encourage the contras to broaden their base and adopt other reforms, including fewer human rights violations.

Some Senate moderates favor a more powerful congressional vote to unleash the remaining funds, and some would prefer that no weapons be purchased with the $25 million that could be spent during the first 90 days.

GOP aides said, however, that neither the White House nor the Senate leadership is willing to compromise on those points and are prepared to absorb however many votes that may cost.