Senate Republican and Democratic leaders, seeking to avoid a bitter partisan fight and a close vote that could send a "wrong message" to Central America, urged a compromise yesterday on aid to the contras in Nicaragua.

But even as GOP aides worked to complete the draft of a proposal the White House hopes will be acceptable to both houses of Congress, key Senate moderates in both parties took issue with at least three major provisions of the plan.

The White House plan would allow immediate purchase of weapons, including portable surface-to-air missles, not require direct negotiations between the Sandinistas and the U.S. government and permit Congress only an advisory vote on releasing the final $75 million in aid.

"All three of those would be problem areas, and they'd have to be substantially modified before the compromise would have substantial broad-based support," said Sen. Jim Sasser (D-Tenn.), the leading Democratic moderate on the issue.

"This thing is just sort of in the beginning, and I don't know whether we're going to be able to work out a compromise or not," Sasser said. "I'm not sure the Republican leadership is a free agent."

A key GOP aide said, "What we have is what the president has offered and we have made no effort to compromise beyond that." Asked if there are any plans to begin additional compromises, the aide said, "No, not at this time."

The Senate is working toward a vote Wednesday or Thursday on Reagan's request for $100 million in military and humanitarian aid to the Nicaraguan contras or counterrevolutionaries..

"It's important to get 70 to 80 votes if we can. It can be more important than 51 votes ," Senate Majority Leader Robert J. Dole (R-Kan.) said yesterday.

Senate Minority Leader Robert C. Byrd (D-W.Va.) said that kind of majority in the 100-member Senate -- which some aides said yesterday is unrealistic -- would make it clear to the leftist Sandinista government in Nicaragua that this "isn't some narrow partisan issue."

A large margin also would boost the administration's chances of prevailing in the House, where an all-or-nothing proposal was rejected last week by 12 votes.

The initial maneuvering by both sides came as White House strategists were looking to fashion an assistance package that they hope would cut short a back-and-forth legislative process between the two houses.

The draft being developed by Senate GOP aides is modeled after an 11th-hour White House alternative that some House members said they would have voted for if it had been legislation rather than an executive order as proposed by the president.

In regard to the newest contra aid package, the administration has said direct U.S.-Sandinista negotiations would undermine the contras, and that immediate military assistance is needed because the contras are constantly under attack from Sandinista helicopters.

Sen. Warren B. Rudman (R-N.H.) added yesterday that if Congress is given too powerful a vote in releasing subsequent funds, "we get to the point where we have 535 secretaries of state and presidents."

Byrd said yesterday that he told President Reagan in a telephone conversation the same day that a strong second vote by Congress was necessary to prod the Sandinistas into negotiations.

Byrd and moderates from both parties say they would prefer no military assistance until after Congress is satisfied that good-faith diplomatic efforts have been made. Byrd also said yesterday that the contras have shown that they are not "in a position to negotiate effectively" with the Sandinistas.

Carlos Tunnermann, the Nicaraguan ambassador to the United States, said, "No self-respecting government would ever negotiate with a group of mercenaries that is directed by a foreign government and that attacks our territory from bases outside."