Only three months out of office as Jimmy Carter's secretary of defense, Harold Brown is not yet of a mind to say precisely to what degree he thinks the Reagan administration's defense program justifies, let alone deals with, the withering campaign charges of American defenselessness.

"I won't rehearse the claims since I didn't find them pleasant then, and I don't now," he says. But that said, the man in charge of the programs that were the main target of the Republicans last year has some interesting and surprisingly upbeat observations about the Reagan administration's approach to defense matters.

Indeed, you come away from an extensive interview with a strong sense that the new Reagan defense budget for fiscal 1982 is not essentially different from the budget that Brown, with a free hand, might have preferred. It is not, in his view, even that much different in emphasis and direction -- leaving aside the big leap in future spending authority -- from the budget that Carter actually submitted on his way out of the White House.

The Reagan budget, he argues, would "build on" the Carter document by allocating more money for a rapid deployment force, buying more spare parts, stepping up production of weapons already on assembly lines and substantially accelerating shipbuilding. But only the $2.5 billion earmarked for a new manned bomber suggests something in the nature of a fundamental change in stretegy -- and that wouldn't materialize before 1985 or 1986.

Meantime, actual defense spending, as distinct from authority to obligate funds for the future, would rise by only about $4 billion in fiscal 1982, Brown points out.

Does this constitute a falling away from the campaign line, or a considered decision to settle for the quick fix of, say, de-mothballing battleships? "It has the look of a quick fix because thath's all you can do in a few months," says Brown, adding: "I don't want to hold them to an impossible standard, any more than I would have liked to be."

The real test of whether there is to be a fundamentally new and different Reagan appraoch -- a determined push for nuclear superiority or for a much larger capacity for military intervention around the world -- won't come until a year from now, Brown figures. That's when the fiscal 1983 defense budget goes to Congress.

Now if that's how it looks to Harold Brown, from the outside, the question becomes: How does it look to the Soviets? Or putting it another way, if you have campaigned for the presidency with the argument that the United States is dangerously weak to deal diplomatically, or military, with the Soviet Union, and almost all you can do in the first year or so is project future heavy increases in defense spending, what do you say once you are in office?

Brown's answer:

"I think what you say is that, because we have started new programs -- even though they may take some time to reach fruition -- and we have made it clear what we are going to be tough, now we can negotiate.

"And in fact, some of the most senior [Reagan administration] officials have actually said that it is not necessary to wait five, six, eight years, or whatever, for new weapons to come into being. We can negotiate after a decent interval."

The question remains whether this will impress the Russians. Is there, in fact, added weight on the U.S. side of the scales, in redressing an alleged imbalance of power, as a consequence or some psychological impact of having demonstrated the intention of building up military strength over the years?

Brown's answer is a qualified yes. He thinks there is a "significant amount of truth" to the notion that intentionss and the education of defense policy by themselves can reinforce the U.S. hand.

But he adds, out of his own experience, that there is as much truth in the argument that "the constant denigrations of U.S. capability by various people made it more difficult to negotiate with the Soviets or to deal with the allies or to do anything."

In other words, the net effect of the new Reagan defense policy may be little more than to effect the damage done by the impression of extraordinary American weakness coveyed by the attacks on the Carter foreign policy/defense policies last year -- attacks that were carried through the transtition and were vividly reflected in the new administration's presentation of its first defense budget.

Or the net effect could be that the Soviets are, or will be, profoundly impressed. Brown claims no clairvoyance on that score. His point is simply that while "attitudes have changed" with the new administration "and some statements have been made about willingness to fight on other people's beches," the rhetoric "still has to be translated into something specific in the way of military strategy" and/or foreign policy. Only then, he insists, will it become convincingly clear just how fundamentally American defense policy has been changed.