Trouble has been coming in bunches for Secretary of Defense Caspar Weinberger. Last Tuesday, The Wall Street Journal, one of the staunchest editorial defenders of his policies, wrote that he is "increasingly perceived to be the man on the fringe in an administration in which his influence once was enormous."

Last Thursday he went through a stormy hearing at the Senate Armed Services Committee, at which such pro-defense senators as Barry M. Goldwater (R-Ariz.) and Sam Nunn (D-Ga.) berated him for being unresponsive.

On Saturday, the president of the United States, with whom he has worked closely for almost 20 years, left him behind in Washington, while other senior national security advisers flew off with Reagan to the Soviet-American summit in Geneva.

On Sunday Weinberger read that a "senior official" on Air Force One had accused the Pentagon of "a blatant attempt to undermine the President," because of the leak the previous day of a letter from Weinberger to Reagan. In the letter, Weinberger urged the president to make no concessions in Geneva that might limit his ability to respond to Soviet violations of previous arms-control agreements. Weinberger denied the Pentagon was the source of the leak, but once again, he was portrayed as being odd-man- out on the Reagan team.

In the midst of all this, last Thursday, I talked with Weinberger at lunch with three other reporters and was struck by the combination of weariness and doggedness in the face he shows the world. His tone was often defensive, as when he said of his exclusion from Geneva by Reagan: "I have no feeling that I'm entitled to a share of his time. . . . I'm here to give him whatever assistance he thinks would be useful." But he was relentless in pressing his arguments for skepticism of the Soviets and full-speed-ahead on the U.S. military buildup -- including "Star Wars" or Strategic Defense Initiative research.

What has happened to Cap Weinberger in the last year has the elements of a classic tragedy. He has been undermined not by his shortcomings but by the excess of his dedication to the mission Reagan gave him.

For those of us who covered Weinberger in his earlier incarnations as budget director and health, education and welfare secretary in past administrations, it has been clear that he approached the Pentagon assignment in a different way from the other jobs. In them, he was skeptical, critical and always independent -- a smart politician deliberately keeping his distance from the bureaucrats he was managing.

At the Pentagon, he has embraced the services' buildup programs and promoted them with the fervor of a convert. It often has seemed as if he viewed the nation's security as a personal responsibility, which no one could or would share with him. The skeptical manager became the uncritical advocate.

Sen. Carl Levin (D-Mich.), an Armed Services Committee member, says, "He definitely has this Churchillian attitude, that somehow he is the savior of Western civilization. . . . Basically, he's cried wolf one time too often."

Levin is a frequent critic, but I talked to four Republican and two strongly pro-defense Democratic members of the Senate and House Armed Services committees, and all of them commented in one way or another on Weinberger's inflexibility.

Sen. Dan Quayle (R-Ind.) says, "He still has a hard-core support group on the Hill, including myself, but there's no doubt that his congressional relationship has deteriorated over the years. He's a tough, disciplined individual, but he finds it exceedingly difficult to compromise, as Congress would like."

When rising deficits and evidence of wasteful procurement practices caused many members of Congress to take a second look at the rapid defense buildup, Weinberger just put his head down and charged. "Congress is truly concerned about the Pentagon process, structure and bureaucracy," says Rep. Jim Courter (R-N.J.), "and those are areas where Cap has not got involved."

"He feels he's morally bound" to his original objectives, Sen. William Cohen (R-Maine) remarks, "and any compromise is at the expense of the national interest in his eyes."

"He just automatically adopted the position of representing the four services," says Rep. Dave McCurdy (D-Okla.). "He never seemed to question them or take charge. Up here, he never would indicate his priorities. It was always take it or leave it."

Despite the damage to his reputation on Capitol Hill, both critics and supporters believe Weinberger will soldier on. Sen. John Warner (R-Va.), one of the supporters, says, "I think the secretary still has that level of confidence that will enable him to carry forth his responsibilities."

That may be so. Reagan's affection for Weinberger is well documented. But others in the White House would not be at all reluctant to throw him overboard.

In his own eyes, Weinberger has kept the faith, while others -- less pure -- on Capitol Hill and in the White House have wavered. He stands guard, battered but proud, over his own dwindling reputation.