Let there be no mistake. Defense Secretary Caspar Weinberger is the strong man of the Reagan administration. He has won another battle over budget director David Stockman and the other members of President Reagan's senior White House staff, sparing the Pentagon's spending plans -- at least for now -- from the cutbacks almost all other parts of government will experience in the budget Reagan sends to Congress next month.

There are many members of Congress of both parties -- including one Republican whose views, for reasons I will get to in a moment, merit special attention -- who think that Weinberger's "victory" may eventually rebound against both the armed services and the GOP's best interests.

But the pattern of Weinberger's success within the administration is so striking and so consistent that it deserves exploration. In every one of the last three years, he has faced massive pressure from the men regarded as some of the most influential of Reagan's advisers -- including chief of staff James A. Baker III, his deputy, Richard G. Darman, and Stockman -- to trim the military buildup in order to reduce the budget deficit.

This year Baker, Darman and Stockman were reinforced in their arguments by all the other members of the Cabinet and by most of the leading Republican senators and representatives, whose help Reagan will need to pass any kind of budget in 1985.

For all their argument that the spending discipline had to be "across the board," Weinberger controlled the only vote that counts: the president's. Once again, the Pentagon gets off with a token cut.

Part of the secret of his influence is surely his long friendship with and service to the president. Part of it is Reagan's own strongly held belief that military power is a good in itself -- not one to be measured against other uses of the money.

But part of it is also the fact that Weinberger has embraced -- more fully than any of his recent predecessors -- the role of spokesman and advocate for the uniformed military services. If Reagan has wrapped himself in the American flag, as critics charged during the last campaign, then Weinberger has put on the armed services' uniform, figuratively speaking, and dared anyone to try to trim it.

Which brings me to that interesting Republican I mentioned earlier. Rep. John McCain (R-Ariz.) is the grandson and son of noted Navy admirals. An Annapolis graduate, he was en route to his own flag rank when he was shot down over Hanoi in 1967 and spent six years in a North Vietnamese prison. After his release, McCain did a tour of duty as the Navy's top lobbyist on Capitol Hill, then retired and in 1982, was elected to a House seat from Phoenix.

A prospective candidate to succeed retiring Sen. Barry M. Goldwater in 1986, McCain is as conservative and defense- minded as his state -- but an intelligent critic of what he sees happening in the Pentagon under Weinberger's management.

In essence, his argument is that Weinberger is not so much running the American military establishment as letting that establishment run him. "In the past," McCain said, "the secretary of defense was the guy who said to the military, 'You can have this much, but you can't have everything you want.' Cap (Weinberger), on the other hand, has almost always endorsed their requests."

What he has not done, McCain said, is provide more than "a feeble attempt" at a plausible rationale in terms of American security interests for the ever-rising Pentagon budget. Under Weinberger, he said, the Pentagon is "very good at explaining the gee-whiz aspects, the virtues of Stealth, B1, MX, the Apache helicopter -- telling how this plane will fly upside down, 900 miles an hour at night, hit the target and come back, and the pilot won't even know he left. But they're not adept at telling why we ever need to send that pilot or aircraft there to start with."

McCain is worried that under Weinberger's stewardship, "public support for significant increases in defense has declined from 70 percent in 1981 to 20 percent now -- without a perceptible change or improvement in Soviet behavior. . . . The American people have lost faith that defense dollars are being spent without 'waste, fraud and abuse.'

But the horror stories about overpriced spare parts are only the "tip of the iceberg," McCain said. More serious is the failure to be clear "about the commitments the United States has in the world" and the reluctance to measure military spending against our own needs and resources.

"What my constituents find hard to understand," McCain said, "is why we still have 250,000 troops in Europe, 40 years after V-E Day, and why we commit 6 percent of our GNP, and Japan only 1 percent, to a defense program that guarantees Japan's oil supply lines."

Unless and until Weinberger answers the questions of the John McCains of Congress, his "victory" must be regarded as shaky.