President Reagan and Defense Secretary Caspar W. Weinberger appear to be on a collision course with Congress over a military budget request so high that even staunch Reagan backers fear that it will undermine the president's goals of cutting domestic spending and avoiding a tax increase.

If Reagan "doesn't really cut defense, he becomes the No. 1 special pleader in town," said Rep. Dick Cheney (R-Wyo.), chairman of the House Republican Policy Committee.

"The numbers cut from defense are not enough, they're not going to do a job, from the budget standpoint," said Rep. Trent Lott (R-Miss.), the House Republican whip who calls himself "a defense person" and, like Cheney, consistently has supported military spending.

Their comments are echoed by others on Capitol Hill and in high levels of the administration who point out that the consensus that backed Reagan's ambitious five-year military buildup in 1981 has shifted fundamentally and now favors a significant slowdown in defense spending.

Resisting this consensus, Reagan last week seemed to be moving toward a Pentagon budget request that would keep the military buildup expanding at nearly the same rapid pace of his first term.

Late last week, Reagan rejected a proposal from most of his budget advisers, crafted by Office of Management and Budget Director David A. Stockman, for a major scaling back of the defense buildup. Instead, the president is expected to announce this week that he has approved only small trims from the budget, as Weinberger suggested.

Stockman wanted to slash $121 billion from Pentagon budget authority over three years; Weinberger has offered to cut $19 billion. Weinberger told White House officials Wednesday that he wanted a budget next year of $316.8 billion, compared with the $284.5 billion in budget authority he has this year, the officials said.

Every time the spending battle has been fought in his presidency, Reagan has sided with Weinberger.

The secretary's determined resistance to any slowdown in the military expansion appears to be motivated by his convictions that more spending is necessary -- although he was once known as "Cap the Knife" when he was budget director in the Nixon White House -- and by a perception that he is pursuing a course that Reagan wants.

Informed congressional sources said Weinberger's key argument to the president was not that defense spending is inviolate. Rather, he appealed to Reagan's instincts as a negotiator and argued that Congress would slash any budget request, even if it were reasonable at the outset. Reagan made precisely the same argument Dec. 6 in an interview with the conservative weekly journal, Human Events.

Weinberger's critics in the administration and on Capitol Hill say this kind of reasoning proved costly to the White House two years ago and would be even more costly now.

Republicans who helped Reagan launch the defense buildup in 1981 now say that support for such increases has dwindled on Capitol Hill because the deficits have ballooned and many other popular programs are on the chopping block.

These Republicans say the consensus now is not to cut back spending, but to hold the rate of the buildup to 3 or 4 percent annually after inflation, much slower than in Reagan's first term, when the Defense Department won an average annual increase of 9 percent.

"I've voted for everything they've asked for . . . ," Cheney said. "Now the severity of the deficit is great enough that the president has to reach out and take a whack at everything to be credible.

"The softer he [Reagan] is on defense, the more he sets himself up for a tax increase down the road," Cheney said. "If you set yourself up to fail on budget cuts, you make a tax increase more likely."

He added that "a package that lets defense run free won't fly. Republicans won't vote for it. Nobody would take such a budget seriously. If you put defense off limits with Social Security and no tax increase, the judgment you've made is that you don't care about the deficit."

"I don't see how you can do that," Cheney said. "If you're going to rule out the other two [Social Security cuts and a tax increase], then you've got to hit defense."

Reagan has been getting this advice from all directions, including in meetings with Treasury Secretary Donald T. Regan and Commerce Secretary Malcolm Baldrige.

It has been voiced by Reagan's closest friend in Congress, Sen. Paul Laxalt (R-Nev.), and his congressional point-men, incoming Senate Majority Leader Robert J. Dole (R-Kan.) and House leader Robert H. Michel (R-Ill.).

"I'm inclined to think he's certainly gotten the message from people he's relied on in Congress that unless there's some give there [in defense], it's kind of unrealistic to expect that you can do the other things," Michel said.

Reflecting sentiments in the Republican-controlled Senate, Dole has been saying for several weeks that Weinberger must come up with "substantial" savings.

The argument also has been voiced directly to Reagan by longtime supporters of the Pentagon, such as Rep. William L. Dickinson (R-Ala.), ranking Republican on the Armed Services Committee. He told Reagan at a meeting last week that he will have "a credibility problem trying to represent the administration in the House if you can't come in with a realistic figure" on defense, a staff aide said.

Reagan also heard concern about defense spending from Republican governors and mayors, such as Cleveland's George Voinovich, who spoke up about it Friday at a White House meeting.

And it has been voiced by Reagan's White House advisers, including chief of staff James A. Baker III, presidential assistant Richard G. Darman and Stockman.

But Reagan, who was elected in 1980 when polls showed that Americans wanted more defense spending, appears to have brushed aside the warnings. He may hear them again next week or next month and reconsider. But he declared in the Human Events interview:

"We're not going to make any cuts in defense spending that are going to drive us backward with regard to what we're trying to do in overcoming the years of neglect in guaranteeing our security."

Reagan's first-term record on military spending exceeded his 1980 campaign promises, which advisers said then would be an increase of about 6 percent a year after inflation. In a series of increases that began even before Reagan took office, defense spending climbed 12.4 percent in fiscal year 1981, 12 percent in 1982, 7.5 percent in 1983, 3.8 percent in 1984 and will rise 5.5 percent this fiscal year.

As deficits mushroomed, Congress pulled in the reins during the last two years. "The bloom has been off the rose for some time," said a key Senate Republican aide. "In 1982, when there was a move afoot to cut defense spending, there were no repercussions in the electorate.

"Public sentiment has changed dramatically," said this official, who asked not to be identified. "Defense spending was not an issue in the [congressional] campaigns this year. Nobody cared how you voted on defense. They asked, 'Are you throwing money down a rathole?'

"No one I know was defeated on how many F18s should we buy, how many nuclear aircraft carriers should we build," he said. "People are now persuaded there is so much waste they are putting pressure on their congressmen -- that's what you're seeing now."

Public opinion surveys have charted this shifting consensus. A University of Chicago poll shows that in 1976, when Jimmy Carter was elected president, 25.8 percent of those surveyed thought "too little" was being spent on defense. It rose to 60 percent in 1980, the year Reagan was elected. It has declined in each Reagan year and this year hit 18 percent.

According to the poll, in 1980, 12 percent thought "too much" was being spent on defense, 27.5 percent thought spending was "about right" and 60 percent said "too little." This year, 39 percent said "too much," 43 percent said "about right" and 18 percent said "too little."

In his reelection campaign this year, Reagan avoided making a commitment to any specific level of defense spending increase. But his unequivocal statements in refusing to trim Social Security or raise taxes turned the heat on the defense budget, particularly with post-election estimates that the deficit would be more than $200 billion a year for each of the next five years without further action.

Going a step further than he had in the past, Stockman after the Nov. 6 election developed the "standstill" defense plan that essentially would freeze military spending authority at existing levels. It would scale back personnel costs and new hardware purchases; Secretaries Regan and Baldrige, among others, supported the idea.

But Weinberger has resisted it at every meeting and apparently has prevailed, meaning that Reagan's fiscal 1986 budget request may be wide of the mark he set of slashing the deficit in half over three years.

"I have real problems with what's going on," Lott said. "I'm a defense person, and my district is heavily impacted with defense spending. I'm torn. I understand what Cap is saying and why he's saying it. But if a credible number on defense is not in the budget, it will not be received well on the Hill.

"In the traditional way of describing it, it will be dead on arrival," he added. "Then we've got a real mess on our hands."