Sen. Sam Nunn (D-Ga.) learned of President Bush's Nov. 8 decision to nearly double the size of American forces in the Persian Gulf when Secretary of Defense Richard B. Cheney called him to a restaurant's public pay phone two hours before the announcement.

Although Cheney is in his job only because Nunn led the Senate in 1989 to reject Bush's choice of former senator John G. Tower of Texas for the Pentagon post, the White House still gives Nunn scant deference.

A White House official says dismissively, "You'd be surprised how little time we spend thinking about Sam Nunn. He loves to have hearings; he loves to hedge. He can make it bumpier or smoother along the way, but in the end he really doesn't matter."

To colleagues in Congress, that appraisal says more about the political acumen of the White House than it does about the influence the chairman of the Senate Armed Services Committee wields these days. "Anyone who doesn't know that Nunn is the vote that counts most up here on the gulf can't count votes," a Republican leadership aide said.

As the Jan. 15 showdown nears for America's challenge to Iraq and Saddam Hussein, Nunn has emerged as the most significant Democratic figure in framing the policy debate, the man one Senate staff member calls "Team B's commander in chief."

Nunn has been there before, but never with the stakes so high or the spotlight so bright. In 1987 and 1988, he challenged President Ronald Reagan's interpretation of the Anti-Ballistic Missile (ABM) Treaty. In 1989, he challenged Bush on Tower's fitness to be secretary of defense. Now, he is challenging the president on the biggest issue yet: the question of war and peace in the Persian Gulf.

It's no surprise to anyone -- including Nunn -- that some people see these as stepping-stones on a path to challenging the president for reelection in 1992.

"Tactically, some of the Republicans have decided that the way to 'get Nunn' is to say he's only doing it because he's running for president," Nunn said in an interview last week. "That's pure bunk . . . and I just have to ignore it."

Two facts help put that statement in perspective.

Fact One: Nunn has been challenging presidents' judgments on military decisions for a long time, starting when a Democrat from his home state, Jimmy Carter, was in the White House and there was no possibility of Nunn replacing him.

Fact Two: Nunn is exploring the possibility of running for president in 1992, but in a characteristically cautious way that leaves him lots of stopping points short of a declaration of candidacy. Many of those who profess to know him well believe caution will prevail over ambition, at least in 1992.

Whatever the case, the 52-year-old former Coast Guardsman, with his accountant's mien camouflaging a competitive instinct that shows most clearly on the golf course, has made himself the leading opposition voice in the domestic debate over Bush's Persian Gulf policy.

Sen. John W. Warner (R-Va.), the ranking minority member of the Armed Services Committee, told the president in a meeting with other congressional leaders of both parties that the administration had goofed in failing to consult with Nunn and others on the November troop decision. It was after that cursory phone call from Cheney that Nunn jumped ship on Bush.

After backing the president all the way on his August judgment to send U.S. forces to Saudi Arabia and his demand that Saddam withdraw troops from Kuwait, Nunn said the new deployments meant the United States was now pursuing "the wrong strategy." In short order, he held a set of televised hearings in which senior retired military officers and former Pentagon officials generally supported his contention that the economic sanctions policy, backed by the threat of force, should be pursued for an extended period before the United States sent ground forces against the Iraqi positions in Kuwait.

While he now thinks that chances of war "have decreased slightly but significantly," Nunn said, "I have to believe that we are indeed preparing for war. . . . At some point, I'd be willing to use military force and would back the president in doing that," but not now and not until sanctions, diplomacy and the threat of force have been exhausted as alternatives.

His concerns range from the prospect of heavy casualties to his belief that even if the United States and its allies defeat Iraq, which he believes they would, "we may be bogged down over there in a continuing kind of long-term ground presence protecting the rulers of Saudi Arabia and Kuwait."

Far better, he said, to scale back the buildup, rotate forces, regain the capability he said has been sacrificed to cope with emergencies elsewhere in the world, and keep up the pressure on Saddam.

By voicing these views in a hundred interviews and showcasing support for his viewpoint from other credentialed defense experts, Nunn has framed the debate.

Far more than when he challenged Reagan on the ABM Treaty and Bush on the selection of Tower, Nunn has appeared in the battle over Persian Gulf policy to be his party's shadow commander in chief. Warner, who is a friend and, often, a partner in defense issues, jabbed Nunn by addressing him during the hearings as "Mr. President," a bit of sarcasm that drew a withering glance from Nunn.

But there is no indication that Nunn is uncomfortable in the role he is playing. Indeed, one could say he has been preparing for it all his life.

He is the great-nephew of the late Rep. Carl Vinson (D-Ga.), the crusty chairman of the House Armed Services Committee, who made admirals tremble and presidents fume, and the successor to the late Sen. Richard B. Russell (D-Ga.), who questioned Lyndon B. Johnson's judgment in escalating the U.S. commitment in Vietnam. Thus Nunn inherits an expansive view of the responsibilities that go with his chairmanship.

"I have a duty," Nunn said, "to try and give my best advice to the administration and to my colleagues. . . . I'm not hesitant to give that advice but I certainly don't think it's my role to dictate or impose on people out there in the field the way they fight the war. That's got to be done by the commander in chief.

"I do think the Constitution makes it absolutely clear that Congress has . . . the prime responsibility on deciding whether we go to war and when we go to war. Just because the Congress hasn't exercised that authority on many occasions doesn't mean to me that we repeal the Constitution."

Nunn said his willingness to challenge the White House on occasion "has been rather consistent over the years," even if "it was not network news" until recently. "I challenged Carter very strongly . . . when he was advocating reduced defense expenditures back in 1976 and 1977, because I thought precisely the opposite needed to be done, because we had greatly weakened our military after Vietnam."

Again, in 1982, when the Republicans controlled the Senate, he was clear in criticizing the deployment of Marines in Lebanon, before the terrorist attack that killed so many in their barracks outside Beirut. The argument he made then is precisely the argument he is making now: that the president has not thought through his strategy clearly enough to give the military commanders appropriate mission orders.

"I pay close attention to military missions," Nunn said, adding that Bush may not have recognized the extent to which his Nov. 8 decisions actually changed the equation for commanders and troops.

Colleagues and admirers such as Sen. Alan Dixon (D-Ill.) say that Nunn is "motivated to do what is right for the country" and is influenced -- in this instance, as in Lebanon -- by the concerns senior military officers express privately to him. Asked about the latter point, Nunn said, carefully, "Most military people . . . would be very reluctant to communicate these views right now, but I do hear things, most of it second- or third-hand, but reliable. I've talked to military people from August on. . . . "

As for the political motivation Republican critics ascribe to him, Nunn readily concedes that he is examining his presidential options but said "I have not gone out and contacted people. I have told everybody that I don't have any plans to run and I am not inclined to run."

He has, however, met with a group of New York financial heavyweights at the home of Iran-contra special counsel Arthur L. Liman and has solicited the views of many top Democrats involved in past presidential campaigns.

Although his public reputation rests almost entirely on his defense expertise, Nunn has signaled a greater interest in close-to-home issues. "The administration has no clue of where they want to take the country domestically and economically," he said in the interview, "and I don't think, frankly, the Democratic Party has a very good response to that. So I am going to be involved in trying to help develop a domestic agenda."

His vehicle will be the Democratic Leadership Council, a group he helped start more than five years ago, which is now headed by Arkansas Gov. Bill Clinton. Nunn said he has urged Clinton to hold domestic policy conferences at locations all over the country and expects to be "out on the road" under DLC auspices.

Meanwhile, he has removed two potential barriers to his candidacy by withdrawing his membership from the all-male Burning Tree Club in Bethesda and by announcing last year that he was ending his past support for restrictions on abortion in favor of "leaving the decision prior to viability to the informed conscience of the mother."

Those who know Nunn's addiction to golf and his native caution universally interpret these moves as signs of heightened presidential ambition. But they say Nunn is a long way from making a decision to run, and may not get there by 1992.

He would be by far the most conservative congressional candidate to seek the Democratic presidential nomination in recent years. Except in 1987, when his backing for Reagan's positions dipped to 47 percent on the Congressional Quarterly presidential support scorecard, Nunn has backed the positions of Republican presidents on well over half the Senate roll calls each year. Last year, he backed Bush 72 percent of the time, fourth-highest of all Senate Democrats.

His CQ "conservative coalition" support scores, measuring the percentage of the time he votes with the majority of Republicans and southern Democrats against the majority of northern Democrats, average about 85 percent in the last nine years.

However much Nunn might have to shift ideologically to compete in the Democratic primaries and caucuses, Georgia politicians say his most wrenching readjustment would be to the bare-knuckle conditions of current politics.

A veteran Georgia politician remarked: "The thing you have to remember about Sam is that he has not been in a real campaign since 1972," when, in the last big gamble of his political life, the then-state representative successfully challenged Carter's appointee to the Senate vacancy created by Russell's death. After winning an upset in the primary, Nunn won the general election by 54 to 46 percent despite the drag of the national ticket headed by Sen. George McGovern.

Since then, Nunn's reelection majority has never been below 80 percent and this year he had no major party opposition. "I don't think Sam has ever had a negative TV ad run against him," the Georgia politician said.

Asked how he would feel about plunging into the contact sport of a presidential campaign, Nunn rolled his eyes in mock horror and said, "That's why at this point in time I'm going to be devoting myself to the agenda and let the candidates take care of themselves."

The transition from legislative to executive responsibility, he said, "would be difficult, there is no doubt about it. The legislative arena is not the same thing as executive leadership." In Congress, "you're basically trying to mold views together, rather than making unilateral decisions."

"That's something I'd have to decide. . . . Is it not only something that I want to do but is it something that I would be good at? Can I really help the nation? Can I really lead the nation? Those are tough questions."

Staff researcher Bruce Brown contributed to this report.