CONCORD, N.H., DEC. 15 -- Former senator Gary Hart (D-Colo.), saying "there is no shame in losing, only in quitting," stunned the political world today by rejoining the presidential campaign he had abandoned seven months ago following reports of his extramarital relationships.

"I intend to resume my presidential campaign and let the people decide," Hart said in a brief speech on the statehouse steps before paying a $1,000 filing fee for New Hampshire's first-in-the-nation primary, to be held Feb. 16. He was accompanied by his wife Lee, and son, John.

"This will not be like any campaign you've ever seen," Hart vowed, "because I am going directly to the people. I don't have a national headquarters or staff. I don't have any money. I don't have pollsters, or consultants or media advisers or political endorsements. But I have something better. I have the power of ideas and I can govern this country. Let's let the people decide. I'm back in the race."

Hart, who had been the front-runner last May when he dropped out, returns as the "compleat" maverick: he faces daunting organizational obstacles, a hostile political establishment, and unanswered questions about his personal life and character.

"This is the worst new idea of 1987," said Rep. Barney Frank (D-Mass.) reflecting a fear in Democratic circles that Hart's candidacy will be another distraction in a topsy-turvy political year in which the surviving Democratic candidates have had a hard time capturing the public's imagination.

But those very troubles, some Hart stalwarts said they think, leave an opening for Hart in the race for the 1988 Democratic presidential nomination. They emphasized that he was running to win; not to register a grievance about what happened to him, or to try to retire campaign debts or qualify for Federal Election Commission matching funds, as some have speculated.

In an interview tonight on ABC-TV's "Nightline," Hart reiterated that he was running to win, then added: "I'm not foolish. I'm not going to perpetuate a campaign that's not going anywhere." But in the same breath, he set forth a goal short of winning, saying that if he could accumulate enough delegates to have an impact on the platform or on other decisions that will be made at the national convention next summer, "that would be a major achievement."

Throughout the day, Hart declined to answer questions about his personal life, saying he had already done so in a painfully and unprecedentedly public way. At an earlier appearance on "Nightline" in September, he acknowledged he had not always been faithful to his wife over the course of their 29-year marriage, including two separations.

"I'm not going to answer any of these questions anymore," he said tonight. "I know what's coming. I'm not naive . . . there will be some bad stories. But I don't intend to respond."

Hart had spoken in recent days with numerous friends and former advisers, many of whom today noted his desire for vindication and his frustration at being on the sidelines. John McEvoy, a former top Hart adviser, said Hart was "bored" with practicing law, and he bitterly accused Hart of "overwhelming hubris and extreme moral blindness."

Former senator George McGovern (D-S.D.), the 1972 presidential nominee whose campaign Hart had managed, said, "There is no question that he's been restless and frustrated . . . . It's not much fun on the sidelines. It is very tough."

McGovern, who has spoken with Hart in recent days, predicted that the campaign would be much like his own 1984 run: little money, little staff, reliance on free national forums such as debates. But Hart aides insisted today that he planned to compete fully and believed he could win.

"We're going to compete in all 50 states," vowed Sue Casey, who helped run Hart's New Hamphsire effort in 1984 and worked as his national scheduler for 1988.

"Gary's not quixotic -- he's a long shot, but he's not quixotic. He understands what presidential politics is all about," said Ned Helms, a New Hampshire Hart supporter from 1984 and 1988 who joined the campaign of Sen. Albert Gore Jr. (D-Tenn.) after Hart dropped out.

Helms returned to Hart Monday night, the only prominent activist in this state to do so in advance of Hart's announcement. More typical was Mark Hogan, former Colorado lieutenant governor and senior adviser to Hart until he switched to the campaign of Sen. Paul Simon (D-Ill.) when Hart dropped out.

"Gary Hart is my friend," Hogan said; "my candidate is Paul Simon." Hart, he said, did not seek his advice, but had he, it would have been stay out. "I think this is going to be pretty unpleasant."

Hart's decision to reenter the race was closely held. He said he made up his mind over the weekend after talking it over with his family, and he and Casey began making telephone calls late Monday night to a small circle of former supporters, most of whom have scattered to other campaigns.

Hart is not going to ask these former staff workers to break current commitments to rejoin him, said Bill Shore, his longtime political aide. Rather, Shore said, the plan is to start campaigning, doing "people events" such as the three-block stroll Hart took down Main Street here that turned into a mob scene of reporters and camera crews. The hope is that the initial burst of free media attention will tap a vein of public support, and that a spartan, largely volunteer campaign organization will follow.

Hart made no mention in his prepared remarks today of the dramatic events that drove him from the race last May, a mere 26 days after he had formally launched his second bid for the presidency. His downfall began when The Miami Herald, acting on a tip, conducted surveillance of his Washington, D.C., townhouse and reported that Hart had spent the night with Miami model and actress Donna Rice, 29, whom he had met at a party in Miami earlier in the spring.

On the night of the surveillance, Hart's wife, Lee, was in their Denver house, nursing a cold. Hart angrily denied the Herald report, saying his relationship with Rice was not sexual and contending that she had departed his townhouse through a back door, unseen by reporters.

But the story landed heavily. In the political gossip mill, Hart had long been plagued by stories that he was a womanizer. They had so much currency that some key campaign staff workers, such as political director Paul Tully and campaign chairman Charles T. Manatt, had solicited promises of good behavior from Hart before they agreed to join the campaign. When the Herald story broke, Hart's support began to hemorrhage: his financial backing dried up, his longtime political allies were mute and his campaign manager, Bill Dixon -- upon learning that Hart had taken an overnight cruise to Bimini with Rice the month before -- quit the campaign.

Within six days, Hart got out, too, taking a shot at the press for trivializing the process, invading his privacy and turning presidential candidates into "hunted" quarry.

Hours before Hart made his decision to abandon the race, The Washington Post had presented one of his top aides, press secretary Kevin Sweeney, with documented evidence of a recent liaison Hart had had with a Washington woman, not Rice.

The evidence included a two-page detective's report of surveillance of Hart from midday, Saturday, Dec. 20, until 8 a.m. Sunday, Dec. 21, 1986. The detective agency was hired by a man who suspected Hart was having an affair with his wife. A confidential source gave the report to The Post the day after The Miami Herald's story was printed. The Post also was given two pictures, purportedly taken by the surveillance team that night, that showed Hart leaving the townhouse of the Washington woman, and of the woman at her front stoop.

The Post independently obtained Hart's schedule for that Saturday, which coincided with the version in the detective's report, as did a number of other facts. The woman indirectly confirmed the relationship on condition that she not be identified.

When The Post's request last May to interview Hart about the matter was relayed to him by Sweeney, Hart, who was campaigning in New Hampshire, reportedly said, "Let's go home," signaling he intended to abandon the race. In subsequent stories, The Post reported on the relationship, but never identified the woman.

Executive editor Benjamin C. Bradlee said today that The Post would continue to withhold publication of the woman's name. "The circumstances under which we confirmed the relationship have not changed."

From the day he quit, Hart, 50, has been restless to find a way back onto the national stage, if not as a full-fledged presidential candidate, at least as someone who could help shape the 1988 policy debate. His longtime friend and supporter Warren Beatty has been among those urging him on. Rick Allen, Hart's former California coordinator, said Beatty was one of those pushing the "If you are going to do it, do it solo" scenario.

"It's the 'Rocky' story," Allen said, "the guy coming off the canvas." Allen referred to this strategy as "the boy and his dog -- he and Lee slogging in the snow in New Hampshire."

Hart's more conventional political advisers counseled against reentry, saying the media -- armed with pictures of him and Rice cavorting on the boat Monkey Business the day of the Bimini trip -- would focus on the sensational.

Last summer, some Hart aides floated a trial balloon, saying that Hart was about to get back into the race. When that drew a negative reaction from the political community, Hart returned early from a vacation in Ireland to deny he had any such plans.

This fall, Hart began making speeches on college campuses, for fees of up to $10,000, some of them drawing far smaller audiences than expected. He used some of the forums to accuse the press of seizing on the sensational at the expense of the substantial in its scrutiny of the character of presidential candidates.

"I don't think the character issue will vanish overnight," said Eric Jacobsen, a UCLA political science graduate student who founded Take Hart America, a Los Angeles-based movement to draft Hart, and who took a red-eye flight to be here today. "The question is, 'Can it be put in perspective?' Will it be broadened to include leadership, or will it be an obsessive focus on one incident?"

Some of Hart's close supporters said one reason he reentered was to respond to a growing demand for his candidacy. But Jacobsen said that, when he made a mailing this fall to about 2,500 former Hart financial supporters, he received only "a few dozen contributions . . . . It wasn't exactly a famous response."

In reannouncing on the statehouse steps here today, Hart returned to the state where he scored the most stunning upset of the 1984 campaign, defeating front-runner Walter F. Mondale by 37 to 28 percent. Weeks before, Hart had been at single digits in the polls.

Casey said Hart would campaign here and in Maine for two days, then fly to Denver, perhaps stopping in South Dakota, which will hold its primary Feb. 23, the week after New Hampshire. He then will spend the holidays assessing the media and voter reaction to his reentry, and make plans accordingly. Hart's daughter, Andrea, who remained in Denver, tonight told Cable News Network she would be the campaign manager. "We've always wanted him to get back in," said John, a college student.

If Hart is to compete for national convention delegates, he must contend with 15 filing deadlines in different primary states between now and Jan 15. And he would have to begin filing slates of delegate candidates in states such as Pennsylvania, Ohio, Illinois, and New York, where the procedure is cumbersome and requires money and a political organization of some kind.

By reentering the race, Hart apparently will have access in January to $1 million in federal matching funds he was seeking based on his campaign's fund-raising earlier this year. When Hart withdrew in May, the FEC had ruled he could not collect the money because he was no longer a candidate.

Hart supporters denied that the renewal of the candidacy was simply a bid to get the money. "Anybody who knows Gary Hart knows that's not what he is about," Helms said.

Indeed, Hart characterized his return to the presidential trail as something of a personal sacrifice.

"Getting back in this race is about the toughest thing that I've ever done and believe me, it's not done lightly," Hart said. "My family is totally behind me. We're in this together because we love our country and we're not quitters . . . . We trust the fairness of the American people."

Hart paid his respects to the other candidates, but then left the clear impression that one reason he has chosen to run again is that he is disappointed in what they have been saying. "When I suspended my campaign last spring, I believed other national leaders would enter the race and I hoped my ideas for reinvesting in the economy, for military reform and for enlightened engagement would be adopted and put forward . . . . Neither of these have happened."

After his speech and his formal filing today -- Hart beat the deadline here by three days -- he headed out along Main Street for some hand-shaking, and promptly got his first taste of life without advance work.

Three times he had to stop and try to organize the hordes of camera crews that pressed so tightly up to him that he could not get to any hands. "Just a three-foot path, please," Hart asked. "Look, you have your job to do and I have mine."

The reactions from passers-by were mixed.

"I think it's his own business who he sleeps with," said Gail Wolfe, who watched the scene with her husband, Don, and their two children. "What he did may not have been terrific, but it shouldn't have knocked him out of the campaign," Don Wolfe said.

The Wolfes, both Republicans, said they would "consider" voting for Hart. Their friend, Tim Cupp, a former Hart supporter who was visiting from Houston, took a more downbeat view: "It's a little late in the game and there are a lot of negative feelings out there."