Before Michigan Democrat Bob Carr lost his congressional seat in the 1980 Reagan landslide, he could feel the defense issue turning against him. Americans were frustrated by the U.S. humiliation in Iran, fearful of Soviet expansion in Afghanistan and ready to support a new U.S. military buildup.

Bob Carr was out of step. During three terms in Congress he had opposed the B1 bomber, criticized nuclear-powered aircraft carriers and generally earned a reputation on the House Armed Services Committee as skeptical of the military brass. "There was a broad, vague image of 'Bob Carr, military critic,' and I think it hurt," he recalled.

Two years later Carr is heading back to Congress after a successful rematch in which he advertised his earlier opposition to Pentagon programs and attacked his incumbent Republican opponent for voting against the nuclear freeze resolution. While the defense issue might not have been decisive, Carr is convinced it played a role in his comeback.

"This time the wind was blowing the other way," he said. "Military spending is not popular with people in my district in 1982. People here are worried about economic survival, and they don't equate military strength just with how much we spend."

Congress rarely makes drastic cuts in defense budgets and so far has made relatively small ones in the budget sent it by this administration.

Nevertheless, the shifting winds mentioned by Carr are part of a new political climate that President Reagan must confront as he seeks continued congressional support for the largest peacetime military buildup in history.

Carr will be in a new House of Representatives containing 26 more Democrats, many of whom appear to lean toward moderation on military spending issues.

Outside the Congress, opinion polls show Americans increasingly fearful of Pentagon spending, which they see driving the government's budget deficit off the charts in an already battered economy.

Some of the most influential powers in the business community have voiced concern that defense spending is "out of hand," and could retard rather than promote a needed economic recovery.

Even The Wall Street Journal, with its solid conservative credentials, has grumbled about the Pentagon as an "enormously inefficient nationalized industry" and has called for "brutally revamping the entire flow of Pentagon investment."

Yet it remains debatable whether any of this will ultimately translate into significant cuts in defense spending.

In the unfolding debate on defense, congressional sources say, the critical factor will be how Reagan and the Democratic leaders in Congress decide they want to be perceived by the electorate in 1984.

Will those who criticize the Reagan defense spending program be viewed as weak, or will they be seen as wise defenders of fiscal responsibility?

Many slash-minded Democrats express fears privately that, in the end, Reagan will win the battle for public opinion with his persuasive television manner and his simple bar charts showing the United States falling hopelessly behind in missiles and warheads.

Since the onset of the Cold War, the American public has tended to see the Russians as more of a threat, or less of a threat, depending on which group of politicians, generals or "defense intellectuals" has control of the debate in Washington. And who has control often has only a marginal connection to what the Russians are doing.

As former ambassador to Moscow George F. Kennan wrote in the November issue of Atlantic, the history of U.S.-Soviet relations over the last six decades "gives the impression that it was not really the nature of any external problem that concerned us but rather something we were anxious to prove to ourselves, about ourselves."

Perceptions, rather than sober evaluations of real needs, have tended to dominate the flow of the debate. And that debate has often been propelled by the pressures of domestic politics rather than substantive issues.

Democrat Jimmy Carter, a dove when he ran for president in 1976, became an advocate of a military buildup in order to combat his image of weakness in the wake of Iran and Afghanistan.

Reagan, the hawkish Republican candidate, opposed the strategic arms limitation agreement negotiated by the Carter administration. But Reagan, the president, has adhered to the agreement's principles.

"This situation has to be handled with care," acknowledges Democrat Bob Carr. "We don't want to position ourselves as the party of defense weakness."

"So little of this has anything to do with substance," says Michael Mawby of the Committee for a SANE Nuclear Policy. "It has everything to do with how the Democrats think the public will perceive them if they hold up big projects.

"A lot of them are afraid Reagan will paint them red just like he's doing to the peace movement."

At this stage, public opinion polls actually are sending a somewhat mixed message.

Between January, 1981, and November, 1982, the percentage of Americans who thought the United States was spending too little on defense fell drastically, from 51 percent to only 16 percent, according to the Gallup poll. But when that small minority is lumped together with the 31 percent who in November thought defense spending was "about right," the result is nearly half the population.

In a Louis Harris poll for Business Week magazine, the percentage of Americans favoring increased defense spending fell more sharply--from 71 percent in 1980 to 17 percent in October, 1982. Twenty-four percent of those surveyed by Harris now want to make cuts, while 54 percent favor keeping defense spending at the present level.

Politicans are also mindful that a strong economic recovery in the next two years could ease fears that the Pentagon programs are bankrupting the nation. Some unforeseen foreign policy flap, especially one involving the new Soviet leadership, could revive hawkish sentiment.

There also is always the workaday impact of old-fashioned pork barrel politics.

The Pentagon is by far the largest governmental provider of contracts and employment, with virtually every congressional district beholden to it for jobs, money and patronage. Even such staunch Senate liberals as Edward M. Kennedy (D-Mass.) and Alan Cranston (D-Calif.) have supported key programs, such as the F18 fighter-bomber, for the engine and airframe business provided to home-based corporations.

Still, there is broad agreement among pollsters and politicians alike that the winds are indeed changing and that leaders ignore this at their political peril.

"It's an extremely dynamic situation," says Jonathan Moore, a former Pentagon and State Department official who now directs the Institute of Politics at Harvard's John F. Kennedy School of Government.

"The electorate is more sophisticated and less intimidated by inflated rhetoric than it once was. The Pentagon's immunity is no longer there.

"People are going to be looking at programs with discrimination and care. There is a requirement for a much more rigorous justification of funding than existed before."

Moore estimates that the constituency favoring serious Soviet-American negotiations, detente and arms reductions has now achieved a rough parity with the group backing a military buildup as the best response to a growing Soviet threat.

Paradoxically, Moore suggests, the Reagan rhetoric may have generated more mass pressure for arms reductions because of the fear it has spread.

"Maybe you could argue at some point in time that events happening internationally required us to be stronger and meaner, but now such events also work in favor of making progress in controlling the balance of terror and stabilizing the international situation," he says.

Whatever the outcome of the spending debate in Congress, some moderates have begun to come out of the closet.

"It's a lot safer to vote against an MX now than it was when sentiment was running the other way," says Sen. Carl Levin (D-Mich.). "The polls are saying that we won't be called anti-defense if we vote for a spending cut or against an MX, a B1 bomber or a nuclear carrier. It's a little easier because the public doesn't connect military spending with strength--it connects it with waste."

"After the Falkland Islands war, people have to wonder why we want to build a multibillion-dollar carrier that can be knocked out by a $50,000 missile," Levin adds.

"They are questioning the waste in the Pentagon and they wonder why our allies are not doing their fair share."

Perhaps mindful of changing public sentiment, some influential members of Congress have been performing an about-face.

Consider, for example, switches executed by Sen. Ernest F. Hollings (D-S.C.), a declared candidate for president in 1984:

* In 1977, Hollings was for the B1 bomber. Now he is opposed, saying it is "too expensive for the mission," and favors spending the money on a new "stealth" bomber.

* In May, 1981, Hollings supported an amendment giving the administration the green light for producing the MX missile. But in September, 1982, he voted to delete production funds until Congress received the president's plan for basing the missile.

* In 1981, Hollings generally supported Reagan on substantial increases in defense spending. This year Hollings wants to reduce the increase from an average 7 percent over the next three years, which Reagan wants, to only 3 percent.

On the other side of the aisle, both Sens. Pete Domenici (R-Ariz.) and Dan Quayle (R-Ind.) have declared that they are open to some cutting of the Reagan program.

In the business world, Chicago investment banker Rudolph Rasin has been speaking out against the Pentagon at nuclear freeze rallies.

"When you see them persist in this ludicrous weapons program you begin to wonder what the hell is going on," says Rasin. "I'm appalled at the magnitude of the spending. The Pentagon is a sacred cow . . . the darling of this administration. I say a bureaucrat is a bureaucrat and just because he wears epaulets doesn't change that."

What is significant about his outburst is that Rasin is a conservative Republican, a Reagan supporter and party activist in DuPage County, a Chicago suburb that regularly gives 75 percent of its votes to GOP candidates.

Unless Reagan can bring the deficits under control, Rasin believes, the economic recovery will stall, the Reagan economic program will be left in ruins and will be replaced by "a New Something" in which the government will manage the economy and "we will be worse off than ever."

Similar sentiments have been voiced by some of the powers in American business and banking.

For example, David Rockefeller, chairman of the executive committee of Chase Manhattan Bank, has declared that "something has to be done" about defense spending.

Robert D. Kilpatrick of the Business Roundtable has expressed "deep concern" about the size of the deficts and has said that spending for defense and entitlements should be cut "even where that is most difficult to do."

However, all of this is still a long way from a "business revolt."

The U.S. Chamber of Commerce's board of directors, for example, is sharply split among supporters of the buildup, advocates of "closer scrutiny" and a group that feels the Chamber should stay out of the fight completely.

"The businessmen don't pose a serious problem for us," says a senior Defense Department official. And critics of the Pentagon tend to agree with that.

"I can't imagine a coalition between peace groups and the budget-busters, because a lot of the peace groups want to transfer the funds saved on Pentagon programs to social programs," says Dina Rasor, director of the Washington-based Project on Military Procurement. "The problem up to now has been that everybody has tended to put ideological purity ahead of building a broad coalition against the Pentagon."

Nevertheless, something is stirring in the American body politic, it seems to Randall Kehler, national coordinator of the campaign to freeze the nuclear weapons buildup.

Although his movement has not lobbied for defense spending cuts, it has spread an awareness of the dangers of an unchecked military buildup, and that has an impact beyond nuclear weapons policy, Kehler believes.

"It's a soft translation, I grant you, from the freeze to defense spending. But there's definitely something new in people's minds."