President Reagan's new defense budget looms as an early casualty of the nation's $100 billion deficits, according to Republicans on the House subcommittee that will recommend how much the military should get in fiscal 1983.

All four Republicans on the House defense appropriations subcommittee agreed that the deficits, threatening to reach $100 billion this year and almost that much next year, put Reagan's record defense requests in an entirely new light.

This view is further evidence that the Pentagon will no longer be off-limits as Congress looks for new ways to reduce federal spending. President Reagan is expected to submit a defense budget calling for $215.8 billion in fiscal 1983 expenditures, up almost $34 billion, or 18 percent, from this year's total.

"The deficit is the best weapon yet," said Rep. Jack Edwards of Alabama, ranking Republican on the subcommittee, in making the point that it will be wielded to hammer down the Pentagon budget.

In an interview, Edwards said the subcommittee "has got to be credible" as it sends the president's new defense budget to the floor against the backdrop of cuts in domestic programs.

That means the subcommittee must cut at least as much as it did last year, about $6 billion, and probably more, Edwards said. He argued that such reductions can be made without cutting into the muscle of national defense.

Rep. J. Kenneth Robinson of Virginia, second-ranking Republican on the subcommittee, said the combination of the deficit and further cuts in domestic programs means "we're going to have a tough time selling Congress on more real growth for defense."

Robinson said he would be surprised if Reagan can get Congress to go along this year with anything approaching an 18 percent total increase in spending and 15 percent in budget authority. "The deficit is going to play a larger part" in deciding how much is enough for the military in 1983, he said.

Rep. Joseph M. McDade (R-Pa.) said he voted for what he regarded as carefully considered cuts of $6 billion imposed by the subcommittee last year only to see them wiped out on the House floor. After that, McDade lamented, the Senate acted like "superhawks" and pushed the House total up by $12 billion, from $196.6 billion to $208.5 billion.

The compromise defense money bill ended up at just under $200 billion.

"We got rolled by the House Armed Services Committee on the floor," McDade said. But this year, he indicated, it should be easier to make the subcommittee's cuts stick. "The engine driving the debate on defense this year is what state we are in with respect to deficits," said McDade, who added that he would like to see cuts of $10 billion to $20 billion in defense.

"I couldn't believe it last year when those cuts were wiped out after we worked so hard," McDade said. He said "there are still a lot of political crosscurrents" that will push against the new defense budget, among them worries about Poland and the Middle East. "I don't know yet which current will prevail," McDade said.

Rep. C.W. (Bill) Young (R-Fla.) said that "as a conservative, I consider a $100 billion deficit frightening," and he said other conservatives feel the same way. He agreed with McDade that the cuts the subcommittee had recommended last year "didn't hurt the national defense at all."

With the added political thrust coming from the deficits, Young predicted that similar cuts this year "should be able to prevail."

But Rep. Jack Kemp (R-N.Y.), chairman of the House Republican Conference, said, "We should not be spooked by the so-called deficit projections." Kemp said nobody is sure whether they will reach $100 billion, and in any case, the defense budget should not be based on deficit estimates.

"I personally don't believe the deficit estimates," Kemp said.

Kemp said he favors closing some military bases and eliminating consulting fees to trim the defense budget this year, but contended it would be foolish to make "wholesale cuts" in response to an economic situation that may not end up being as dark as some are portraying it.