out of a self-made political shell that has often insulated him from controversy during his 12 years in Washington.

This fall a newly assertive Nunn has derided President Reagan on network television and challenged years of conventional wisdom on military policy. Yesterday, with Sen. Barry Goldwater (R-Ariz.), Nunn presented a plan frontally attacking many of the Defense Department's most ingrained habits and calling for a sweeping overhaul of the Pentagon.

Nunn surprised many who know him well when he turned on Reagan in a recent television interview program. Asked about Reagan's contention that the United States was inferior to the Soviet Union in "literally every kind of offensive weapon," both conventional and strategic, Nunn replied:

"I don't think that's correct. I think the president needs to sit down with the Joint Chiefs of Staff and learn about our submarines, about our aircraft carriers, about our tactical air, about our cruise missiles, about our bombers and other advantages. It's important for the president to know where our weaknesses are but it's also very important as we lead into the summit for him to know where we're stronger."

While building a reputation as one of the most knowledgeable members of Congress on defense issues, the moderately hawkish Nunn has rarely been inclined to tangle with presidents. Nor, in the view of many who know him, has he had an impact on the defense budget and the nation's defense capability commensurate with his expertise and reputation.

"I just wish he'd speak out publicly like that more often," one Democratic senator said after Nunn took the shot at Reagan on television. "He could defeat something like 'Star Wars' the president's plan to develop a strategic missile defense system single-handedly if he'd really come out strongly against it. Very few of us understand it, but he does, and he could put it on the back burner."

After several years of trying to get the Pentagon to establish its priorities and match its capabilities to its commitments, Nunn is taking the lead in a new Democratic attempt to devise a defense policy that separates the Democrats from Ronald Reagan's expensive defense buildup, but doesn't look "soft" on defense.

An important part of Nunn's argument -- contained in the Senate Armed Services Committee staff report released yesterday -- is that deficiencies in military organization and practices have offset many of the benefits of the Reagan buildup.

Nunn's credibility, colleagues say, is based on his mastery of the many complex and arcane facets of defense policy, his centrist instincts, his moderate but well-chosen rhetoric and his sense of timing.

"He waits until he has the high ground of facts," says Jeffrey Record, a former aide to Nunn and now a senior fellow at the Institute for Foreign Policy Analysis.

Nunn has used his influence sparingly. He has lobbied the Reagan administration to work seriously toward arms control, and had a meeting this summer in Moscow with Soviet leader Mikhail Gorbachev, which persuaded him that a deal is both possible and desirable. He has long worked to improve U.S. and NATO conventional forces, and has criticized overreliance on nuclear weapons, particularly in Central Europe.

"We still would have to use those tactical nuclear weapons in the first day or two of a conflict with the Soviets even though we've poured billions into conventional defenses there," Nunn said in an interview. "So what's the point of keeping 300,000 troops over there? We need an explicit agreement with the allies in Europe. There's no reason for us to spend $180 billion a year there if they're not serious."

Out of this frustration last year he offered the "Nunn amendment," which would have dictated the withdrawal of up to one-third of the 326,000 U.S. troops in Europe if the other NATO members didn't fulfill their commitment to stock a 30-day ammunition supply and provide security for 3,000 NATO aircraft. It narrowly failed to pass the Senate but he got his message across: the NATO allies have pledged to spend $12 billion over five years to fulfill these commitments.

Nunn is now trying to counter what he sees as Reagan's disproportionate buildup of strategic weapons -- including the Strategic Defense Initiative (SDI) -- which he believes is working against a significant arms control agreement with the Soviets.

"I'm concerned about the impact of our nuclear programs and the SDI on our conventional forces in a time of declining growth," he said. "We're tilted much too far in that direction and there's a lot of waste . . . . "

Nunn has expressed deep skepticism about SDI, once describing Reagan's vision of a potentially impenetrable shield against Soviet missiles as "bordering on the absurd."

"If you, in effect, eliminate nuclear weapons, that's the ultimate, it's better than arms control and if you believe you can do that, there's no reason to compromise, there's no need for flexibility at Geneva," he said. "But it a shield against missiles doesn't apply to bombers or cruise missiles and terrorists could still put a suitcase nuclear bomb in a basement in every city. It doesn't make nuclear weapons obsolete. Or biological warfare."

Yet Nunn helped orchestrate Senate passage of a compromise providing $2.96 billion for research on SDI this fiscal year. (A House-Senate conference cut the figure to $2.75 billion.) He defends that position by saying the research program is a valuable bargaining chip with the Soviets in arms control talks.

Such episodes have raised questions about Nunn among students of defense issues, Senate colleagues and Capitol Hill staff members. In a series of interviews, many questioned whether Nunn's effectiveness has been proportional to his reputation and whether defense policy and the national security are appreciably different because of his presence.

They cite two of Nunn's major positions -- his espousal of a modest (3 percent a year) but steady growth in defense spending rather than the familiar cyles of "boom or bust" appropriations, and his skepticism about the use of tactical nuclear weapons in the defense of Europe. Neither have fared well in Congress, despite Nunn's efforts.

"Nunn can't carry the Democrats in the Senate," Record says. "But he has saved the bacon for several presidents."

Some Democrats are skeptical of Nunn partly because he has supported Reagan on key votes more readily than almost any other Democratic senator. His support ranges from 58 percent in 1981 to 69 percent in 1982 and 1984, and he has supported Reagan 66 percent of the time so far this year.

"Sam is often critical of Reagan but he always votes for him," one Democratic senator said. "After all, he's from a state that has lots of defense facilities and he's here turning on the faucets as much as anything."

A former aide to the Senate Armed Services Committee agrees: "Always before, Nunn's demands were quantitative; he just wanted to spend more. Now he says the Democrats should stand for something. Now he's concerned about the quality of defense as well. Where was he when we needed him?

"Back in 1979 he could have gotten any defense program out of President Jimmy Carter he wanted in exchange for his support on SALT II strategic arms treaty , in addition to the spending increase he pushed for. Still, in the last couple of years there's evidence that he's trying. We all live and learn."

Only in recent months have the Democrats been able to markedly alter the Reagan defense buildup, after more than four years of going along with the president, or failing when they tried to challenge him.

"They just can't bite the bullet and vote against something like the MX missile ," said Jeremy J. Stone, director of the Federation of American Scientists. "I find them embarrassing to watch. They're like a grandfather who always grumbles but always hands over the money. They always find a reason, like trading their votes for MX for the president's promise to seek a meaningful arms control agreement."

But the times may be changing.

"Until the last six months or so the Republicans had more unity and cohesion on defense than the Democrats," Record said. "But this has come unraveled this year because of the budget deficits and their deficit phobia."

"The next four or five years will be the toughest in a long time because of the economic problem," Nunn said, in a reference to new Pentagon budget constraints. "We're trying to force them to sort out their priorities. The services have run the show in this administration and have gotten almost everything they wanted," he said.

Nunn has specialized in defense issues since he was first elected to the Senate in 1972. He came by this interest naturally.

The late Rep. Carl Vinson (D-Ga.), a longtime chairman of the House Armed Services Committee, was Nunn's great uncle. Nunn inherited the Armed Services Committee seat of the late Richard B. Russell (D-Ga.). Sen. John C. Stennis (D-Miss.), Russell's successor as Senate Armed Services Committee chairman, gave Nunn opportunities to play an active role. Nunn was the protege of the late Sen. Henry M. Jackson (D-Wash.). Today he is regarded as more knowledgeable on detail than Russell and more flexible and open-minded than Jackson.

During Nunn's first years in the Senate, Democrats like Stennis and Jackson were opponents of their more numerous liberal Democratic colleagues. But today those divisions are much less sharp. Time and a changing climate of opinion in the party are helping to close the gap between Nunn and the liberal wing of the Democratic Party. Democrats now acknowledge that they need a defense program of their own, and Sam Nunn is trying to provide one.