Willingly or unwillingly, rightly or wrongly, Defense Secretary Harold Brown has now politicized himself and the Pentagon more than any of his predecessors. And there is considerable risk that any short-term political gains Brown has made for candidate Jimmy Carter will be far outweighed by the long-term losses, not only to President Carter but to the whole process of arriving at rational decisions in the all-important field of national security.

For no matter what you may think of Harold Brown, he is the one and only man at the top of the Carter administration who really knows what he is talking about when it comes to the modern weaponry that threatens everybody on the planet. As a brilliant nuclear physicist, he helped develop the intercontinental ballistic missile and the hydrogen bomb and has wrestled with the Soviets on arms control issues for more than two decades. Harold Brown is truly an expert witness, as both the executive and legislative branches have come to appreciate in his first tour as steward over the world's most complicated military extablishment.

Yet the cutting edge of this expertise has been dulled and chipped because Carter and Brown have used it too much in the political campaign. The defense secretary's credibility as a non-political, expert witness on such questions as the verifiability of the pending SALT II treaty has been eroded -- at least for the short term. If Carter is reelected and keeps Brown as his leading SALT II advocate, any loss in Brown's credibility will be loss to the president as well.

Although Brown is far from the first defense secretary to involve himself in a presidential campaign -- as former Republican candidate Barry Goldwater has testified, in complaining that former defense secretary Robert S. McNamara politicized the Pentagon against him -- he has dug himself deeper into it than any of his predecessors.

Last week, Brown was exchanging political fire directly with Ronald Reagan on the issue of whether Brown publicized secrets about radar-invisible Stealth aircraft to help combat Republican charges that Carter has been soft on defense. Shortly before that, even though his official spokesman had said Brown was going to the Democratic National Convention only to listen to Carter's acceptance speech, the defense secretary went on the convention floor to lobby for the MX missile.

It was also Brown who gave the one and only public explanation of Carter's new and controversial nuclear targeting doctrine, again because of his expertise; who attacked the defense planks in the Republican Party platform; who took the lead in defending the disastrous Iranian rescue attempt made under a plan he had personally reviewed and endorsed; who accused previous Republican administrations of cutting the defense budget, despite the fact he had bragged early in his term at the Pentagon about cutting the inherited Ford budget; who invited the press to come along to White Sands, N.M., to watch testing of the cruise missile -- the weapon Carter had chosen over the B1 bomber.

Brown denies that all this adds up to politicizing himself and the Pentagon. He says he regrets that national defense has become a central issue in the presidential election campaign. But now that it has, Brown contends, he has an obligation to set the record straight for the American people, allies abroad and potential enemies.

"I consider it part of my responsibility as secretary of defense to try to make the American people understand the facts on important defense issues," Brown said in justifying his responses to Reagan's charges on Stealth disclosures. He contends it is important for everybody to understand that the United States is still the most powerful nation on earth; that American technology is still keeping the country ahead of the Russians, with the Stealth breakthrough in making planes invisible to radar only one case in point. "I think it is part of my responsibility to make it clear what U.S. advantages are." One obstacle keeping people in the United States ans elsewhere from making informed judgments about the state of the national defense "is an unwarranted denigration of U.S. capability. This hurts U.S. security."

Brown, a somewhat introverted man, sees himself as one who has been dragged onto the center of the political stage by events. He says he is not enjoying the starring role -- a larger one than the extroverted ex-pol, Melvin Laird, played before him as a Republican secretary of defense.

But is is not only Reagan who is complaining about Brown's going political. Other critics include both Democrats and Republicans: Republicans on the House Armed Services Committee who insist Brown went out of bounds for a defense secretary by holding a televised press conference on Aug. 22; senators who claim he has been playing politics for Carter with the defense budget; and active and retired military people who fear Brown and some of his service secretaries have politicized the Pentagon for all time.

Perhaps such criticism will blow away along with all the smoke of the political campaign after the November election. If not, the administration's only fully certified expert on the life and death questions of nuclear arms will have lost credibility at a most crucial time.