The Reagan administration is preparing to ask Congress for $30 million to $50 million in renewed military aid to rebels fighting the leftist government of Nicaragua, despite Republican warnings that it may not be approved, congressional and diplomatic sources said yesterday.

Officials will also ask for a renewal of and increase in a $27 million nonmilitary aid program that expires at the end of March, the sources said.

In an interview, Elliott Abrams, assistant secretary of state for inter-American affairs, said he believes that the "votes are there" for military support for the rebels, although he declined to comment on whether or when the request might be made.

He noted that last year's furious debate over nonlethal aid has yielded to semiautomatic support for it and that members of Congress are increasingly critical of the Sandinistas governing Nicaragua.

In fact, he said, the Sandinistas have become "the administration's unpaid lobby" against themselves by their recent efforts to curb criticism from the Roman Catholic Church and dissident Nicaraguans. "I'm optimistic," he said.

A well-placed Senate source said Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) has warned the White House that the Senate will reject any effort to revive a covert military program run by the Central Intelligence Agency. The administration is expected to seek that.

However, Lugar is understood to have added that the Senate may well approve an open military-aid program if the administration and counterrevolutionary leaders make a strong public case, possibly through Senate hearings being urged by Lugar, that the contras are firmly democratic and worthy of the aid.

Such a move would at least tacitly recognize a Nicaraguan government-in-exile, which the administration has been reluctant to do.

Rep. Dave McCurdy (D-Okla.), key to administration success in arranging the $27 million nonmilitary aid package last year, said that the program could be renewed or increased but that military aid would be rejected. He added that a great deal depends on administration timing.

"The situation can change a lot between now and when the vote is held," he said.

The administration has been unhappy with the humanitarian-aid program, which has had trouble making deliveries and embarrassed the Honduran government.

Contra attacks on Nicaragua are launched from bases in neighboring Honduras, whose officials helped the CIA transport covert aid to the contras before Congress ended that program in 1984. Honduras has always denied that rebels use its territory.

One planeload of overt U.S. aid, mostly medicine and clothing, reached the rebels last October, but Honduras blocked a second shipment and no others have been delivered. Instead, the rebels are making purchases in Central America, and the Nicaraguan Humanitarian Aid Office in the State Department is paying the bills, according to officials here.