President Reagan's drive for aid to the contras in Nicaragua takes center stage in the Senate this week -- its prospects boosted, but far from guaranteed, by partisan and tactical advantages it lacked when the Democratic-controlled House rejected it four days ago.

The sharp differences, intense rhetoric and search for an acceptable alternative to the defeated all-or-nothing proposal lingered over the weekend, and reaffirmed that the issue is likely to be resolved neither quietly nor immediately.

Key Democratic and Republican senators predicted again yesterday that some military assistance will be approved by the GOP-run Senate before it adjourns for Easter recess. The measure is expected to come to the Senate floor late Tuesday.

Even with a Senate victory, "the real battle" for contra aid "will come once again in the House on April 15," Sen. Jim Sasser (D-Tenn.) said on NBC News' "Meet the Press." White House chief of staff Donald T. Regan was nevertheless optimistic. "We'll win again in both places," Regan told ABC News' "This Week With David Brinkley."

Already, however, disagreements are developing between the chambers over whether a Reagan-fashioned aid package approved by the Senate and sent back to the House would take precedence over any House-crafted alternatives still in preparation but scheduled to be acted on April 15.

Moreover, pivotal Senate moderates in both parties take issue with the policy -- or, some complain, lack of one -- implicit in the latest version of the White House request for $70 million in military assistance and $30 million in nonlethal aid for the Nicaraguan counterrevolutionaries, known as contras.

Vote counters in both parties show Reagan ahead, but with 11 or more senators undecided.

To ease the going in the Senate, the White House has indicated an early willingness to make modifications it shunned until the final hours of the House debate.

Essentially, Senate GOP leaders will try to cast in legislative form the contents of an executive order that Reagan, in an 11th-hour appeal, told the House he would issue if the spending package were approved. Some House members said they would have voted for the plan if the provisions of that order had the force of legislation.

White House strategists also indicated that they will abandon the partisan, strident anticommunist rhetoric that backfired in the first House vote and drew the ire of Sen. Nancy Landon Kassebaum (R-Kan.) as "simplistic reasoning."

These changes are expected to make it easier to begin rallying behind the president what many on Capitol Hill accept as a clear bipartisan majority in each chamber favoring military aid but divided on what limits should be placed on it.

Despite the White House moratorium on sharp tongues, the president said in a New York Times interview published yesterday that unnamed opponents of his plan made "scurrilous, personal attacks against me, for example, the most dishonest use of distortions and outright falsehoods that I have heard in a legislative battle."

Rep. Thomas S. Foley (D-Wash.), the House majority whip, said the accusation was "incomprehensible" and accused Reagan of "posing here as a victim of harsh rhetoric when indeed the harsh rhetoric is all in the other direction."

"The fact of the matter," Foley said on the Brinkley show, "is that this president has had and enjoyed -- and I think a lot of people feel that it's a healthy turn of events -- great public popularity and absence of any personal criticism. But no president has a right to expect an absence of criticism of policies of his administration as a mark of personal respect."

Many of the same doubts that were voiced before the House defeated the Reagan proposal, 222 to 210, remain in the minds of senators.

Some members, for instance, simply do not trust the administration to pursue good-faith efforts to negotiate changes that would encourage more democracy and greater human rights in Nicaragua, and simultaneously lower the Soviet presence there and stop the leftist Sandinista government from exporting revolution.

Instead, they say the administration is bent on overthrowing the Sandinistas, using a loosely organized band of unproven combatants, tainted by ties to the repressive regime the Sandinistas threw out and without support from Nicaraguan neighbors, including the democracies.

As a result, some GOP senators would like to see stronger White House efforts to meld the groups opposing the Sandinistas into a broader-based organization before any money is spent.

But Democrats such as Sasser maintain that 90 days is not long enough to pursue diplomatic efforts conclusively and that the administration is unrealistic to expect the Sandinistas to talk with the contras without bilateral U.S.-Nicaragua talks -- which administration aides say would undermine the contras.

Many Senate moderates, moreover, do not accept Reagan's case for the urgency of immediate military aid. White House aides say that point is nonnegotiable.

Under the latest Reagan proposal, $25 million would be spent right away, much of it to purchase portable surface-to-air missiles for use against the Soviet-made attack helicopters vital to the Nicaraguan government's fight against the Honduras-based contras.

These moderates, along with some more liberal senators, lean toward "fencing" any military expenditures until after congressional certification of earnest diplomatic efforts -- something not in the latest White House plan.

The 53-to-47 GOP edge in the Senate makes the White House burden more one of holding its own members than converting those in the other party, a task it faced in the House, where 16 of 180 Republicans voted against aid.