Sen. Richard G. Lugar (R-Ind.), new chairman of the Foreign Relations Committee, said yesterday that Congress likely will deny further aid to the antigovernment "contra" guerrillas in Nicaragua.

Lugar, who has supported the aid, said the program no longer is "viable."

Lugar's assessment is the latest of a series of signals to the Reagan administration from Republican and Democratic lawmakers that an expected request for new aid faces difficult if not insurmountable obstacles in the 99th Congress.

But Lugar's warning was especially significant in light of his previous support for the aid and his general record as an administration loyalist.

Speaking of the once-covert program that has become one of the most highly publicized foreign policy issues in Congress, Lugar told the National Press Club: "Aid to the contras is not viable because it is no longer covert and because Congress will probably not continue to fund it."

An earlier Senate Republican blow to the program came late last year when Sen. David F. Durenberger (R-Minn.), new chairman of the Senate Select Committee on Intelligence, came out against further funding of covert activities, although he left the door open to overt aid.

Lugar went further yesterday in indicating opposition to overt assistance. "That would be very close to declaring war, and there's no consensus in the American public for that," said Lugar.

Instead of such aid, Lugar said, the United States should find new ways to keep pressure on the leftist Nicaraguan government to make peace with El Salvador and other neighbors and to resist the establishment of Soviet bases within its borders.

In an interview with the Associated Press yesterday, President Reagan said there would be "great difficulties" in openly aiding the contras "because in the world of international law you find that you changed the situation completely, and you then find yourself having to weigh what are then considered acts of war."

Although the Republican-controlled Senate generally has supported financial aid for the rebels, it joined the Democratic House late last year in voting to suspend aid until another vote is taken after Feb. 28.

The administration repeatedly has indicated that it will ask that the ban be lifted. The Central Intelligence Agency then could use $14 million to aid the contras during the fiscal year ending Sept. 30.

On other issues, Lugar said the Reagan administration should find ways to pressure the white South African government to share power with the country's majority black population.

While declining to say whether he would support sanctions against South Africa that Sen. Edward M. Kennedy (D-Mass.) and others are expected to push, Lugar said the issue will be debated seriously.

In a warning to South Africa's minority white government, Lugar said its officials will be making an "awesome mistake if they do not reform and do not reform quickly.

"It is apparent they are headed for a bloody civil war which would be a disaster for the western world," he added.

But Lugar said he had misgivings about sanctions because it is not clear whether they would lead to improved conditions for blacks or deeper troubles for them.

While parting company with the administration on policy toward Nicaragua, Lugar came out strongly for the administration's "Star Wars" space defense program and for a continued military buildup while arms control negotiations proceed.

He said the space-weapons initiative was responsible for bringing the Soviets to the bargaining table, and added, "We should be prepared for the fact that this research effort is here to stay and that its consequences will be great."

On defense spending, Lugar said, "Arms control arrangements can affect the pace and character of some military programs, but they cannot alone contribute substantially to the balance we require for our security."

Implying opposition to a defense spending freeze under study by Senate Republicans, he suggested that it would hamper U.S. efforts to get allies, including the North Atlantic Treaty Organization, to increase financial support of joint military efforts.