DENVER, DEC. 19 -- Ronald Reagan and Mikhail Gorbachev had just signed the INF Treaty, and Denver television anchorman Ed Sardella decided to ask a local foreign-policy expert to come in and discuss the pact. Gary Hart readily accepted the invitation; he strolled over to the station, alone and unnoticed, for his few minutes before the Channel 9 cameras.

When the Dec. 8 newscast ended, Sardella asked Hart how things were going: "Are the media finally leaving you alone?" "Oh yeah," Hart replied sardonically. "Maybe a little too much."

Precisely one week later, Hart stood before a battery of network cameras on the steps of the New Hampshire state capitol and solved that problem. With five simple but stunning words -- "I'm back in the race" -- Hart put an end to what his friends describe as the gnawing, maddening frustration he felt watching candidates he considered his inferiors playing in the big game while he was confined to a dark spot in the bleachers.

But Hart's reentry reflects more than just a politician's instinctual love of the limelight. With his new one-man-against-the-world campaign, Hart has brought to life a personal dream for winning the White House that he nurtured throughout nearly eight months in political Siberia. It's a dream he was talking about within hours of his fiery withdrawal from the Democratic race May 8.

"We were out in {Hart's Colorado home in} Kittredge two days after he withdrew," a former campaign worker recalls, "and Gary was talking about a much different way to win . . . a 'light infantry' campaign, no gray eminences from Washington, no press conferences, just Gary making speeches. He had it all figured out . . . . I thought he might mean 1992 or '96."

In Washington and elsewhere, many dismiss Hart's new, Phase Two campaign as a hopeless gesture, a quixotic exercise. That it may be, but Hart stepped back into the race only after looking long and hard at the other Democratic contenders and concluding that he could beat any of them, defying the conventional wisdom that the heavy baggage of last spring's personal scandals will doom his candidacy. Hart's thesis is that his campaign, centered on the candidate's anti-media, anti-Washington message, will give the Coloradan a chance to travel the country attacking his personal nemeses, the press and the "power brokers" -- and to win the Democratic presidential nomination in the process.

"One thing I can tell you," said Dennis Walto, a young campaign aide who has talked to Hart repeatedly in recent weeks, "Gary wouldn't be doing this if he thought anybody else was in a position to win.

"But he came back from Boston a few weeks ago and told me point-blank that {Massachusetts Gov. Michael S.} Dukakis will not win New Hampshire. He thought for a while that {Illinois Sen. Paul} Simon would come roaring out of Iowa and take over, but now he doesn't think that will happen."

"He was very level-headed about it, both the plusses and the minuses," said Kevin Sweeney, the former campaign press secretary who discussed the reentry at length with Hart two weeks ago. "This is not Don Quixote or anything. He's running because he thinks he can win it.

"The basic formula for Gary is, it only takes 20,000 votes or so in New Hampshire to get you into the semifinals -- 'Super Tuesday' and then Illinois, Ohio and all that," Sweeney said. "Okay, he gets into the semifinals with this campaign he's got now, a referendum on the media, a referendum on the system. And if he gets that far, he thinks that gets all the personal stuff out of the way and he can just go to voters with his ideas and his intellect."

John Emerson, a Los Angeles lawyer who has been talking to Hart ever since last spring's debacle, said the harsh attacks from columnists and other candidates that greeted Hart's reentry play neatly into this game plan. "Gary thought -- in a way, Gary hoped -- he would get in a situation where everybody was against him but the people. So when all these voices out of Washington say he has no right to get back in, that fits perfectly with what he wants to do now."

Another facet of Hart's deliberate decision-making is his sense that rules and obstacles that hinder other candidates do not apply in his case.

"I was talking to Gary just after {Colorado Rep.} Pat Schroeder decided not to run," said Colorado Democratic chairman Buie Seawell. "I said you couldn't really blame her, because it's so hard to organize and get on ballots and slate delegates and all that when you start late. And Gary said, 'No, it's not that hard. You could do all that.' "

Hart's friends and family say he did not reenter politics as a way to escape an unhappy life here in Denver. According to colleagues at Davis, Graham & Stubbs, the law firm where Hart is paid something over $100,000 a year, he worked hard at the firm, advising both business clients and fellow lawyers. He had been assured that his "of counsel" status, in which he is expected to work about half the hours of a full-time partner, would be continued as long as he liked. Hart has contracts for a novel and a nonfiction book and has been working at both.

Those who have talked with him said he felt happy with the progress of his lecture tours -- so much so that he had printed at his own expense a collection of his lectures, titled "Reform, Hope, and the Human Factor." In addition, family friends say, Hart's marriage is strong -- almost as if his wife, Lee, wanted to spite all those who predicted she would soon seek a divorce. "The two of them reached an accord on the marriage," said Larry Lawrence, a San Diego hotelier who has known the Harts for two decades. "They had a feeling that all they had was each other."

But none of that could offset the crushing frustration Hart felt at being out of the presidential race. "He got over his anger," said his daughter, Andrea. "But he's been basically frustrated. Here is a man who has been conveying his ideas to a national audience -- and to have to get out and sit back with no role."

Over the summer, as Hart, his family and his staff loyalists tried to adjust to life outside of politics, that frustration came to the fore in countless discussions, all coming back to the same question: Did Hart make a mistake when he dropped out of the presidential race?

"By summer, we were all thinking pretty hard about the initial decision," recalls Sue Casey, a longtime aide who has become de facto campaign manager of the new Hart drive. "Could we have weathered the storm? If Gary had just toughed it out, wouldn't he still have a good chance at the nomination today?"

Within the family, both Hart children -- Andrea, 23, and John, 21 -- argued forcefully that Hart never should have withdrawn. Lee Hart's position was that news reports on Hart's relationships with women were so painful and damaging that he had to leave the race to end that kind of publicity. Gary Hart, friends say, saw merit on both sides.

Still, the candidate and his advisers say he was completely surprised last August at the sudden rush of rumors that he might get back in the race. Hart cut short a vacation in Ireland to scotch that prediction. But in doing so, he heard from several friends that he should indeed get back in.

"There were detailed, detailed scenarios around," said Beth Smith, a veteran Hart aide who had moved to Denver to work on his presidential campaign. "It was sort of a given that Gary would run for president again. Basically, the money people were all telling him to run now, and the political people were telling him he ought to wait until 1992 or '96."

"The concept I was pushing," said Lawrence, the San Diego millionaire who has long been one of Hart's fund-raisers, "was that you get back in with a total nonorganization campaign, just the candidate and two advance guys. And I knew Gary liked that concept because that's exactly the image he has, that he works best as a loner, when he's up against everybody.

"Gary said if he did do that, it would redeem him with his children," Lawrence continued. "But Lee was against it, because she was worried about the press and the criticism he would get. By Thanksgiving or so, I thought it was pretty clear that there was a 3-to-1 vote in the family. But Gary wouldn't do it unless Lee came around."

On Dec. 5 -- 10 days before his surprise appearance at the New Hampshire capitol -- Hart had breakfast in California with three of his 1988 campaign aides. "He had everything figured out pretty clearly," said Sweeney, who was present. "On the positive side, he thought he had a good chance to make the semifinals and then go on and win the nomination. And if that didn't work, at least this time he would go down for political reasons, not the personal stuff.

"The downside was that the publicity could be gruesome, the pictures and the rehash of Donna Rice."

Hart said at that breakfast that he fully expected a negative reaction if he reentered the race. But he thought it would be a "30-day bashing," at which point the news media would move on to the next story.

The next day, Hart went for a long drive along the California coast with Walto, the young staffer. "He had the race pretty well figured out," Walto said. "Not every detail, but he could definitely see a way that Gary Hart could win it."

Sue Casey said Hart came home from California and asked his family again about the wisdom of reentry. Gradually, Lee Hart came around.

"Watching Reagan and Gorbachev on TV had a lot to do with it," Casey said. "Gary was thinking, 'I can deal with this man.' "

Last Sunday, as the Hart family sat down for one more discussion of the same old question, somebody pointed out that the deadline for presidential candidates to file in New Hampshire was just five days away. "I think that put it over the edge," Casey said. "There was no more time to talk about it. It was a red-alert kind of thing; either you do it now or you'll never be able to."

That night, Hart began calling friends and campaign workers: "Lee and I have decided to get back in. The initial idea was a quick strike into Iowa followed by a flight to New Hampshire to file as a candidate. But a big snowstorm in Denver snarled that plan, so the Harts went straight to Concord.

Over the next three days, Hart took his "no-frills" campaign to New Hampshire, Maine and South Dakota, staying at friends' houses and making up the schedule on the fly. It was an impossibly makeshift, seat-of-the-pants presidential campaign, and Hart was clearly loving it.

"This is like the start back in '83," the candidate said, recalling the days when he was an unknown also-ran seeking the presidency. "You just campaign all day and then find a floor to curl up on for the night." And then Gary Hart threw his head back and laughed the laugh of a happy man.