GARY HART'S RETURN from his seven-month sabbatical has produced a stunning and delicious paradox: almost no one takes his candidacy seriously -- except the 30 percent of Democrats who make him the front-runner in all of the latest national polls.

How can this be? Does Hart know something about the electorate that the political insiders he's running against miss? Is there a ready-made bloc of votes out there to be had from "everybody who's ever made a mistake," as Hart now describes his constituency?

Or are Hart's poll numbers a false barometer -- even a snare?

The answer is almost surely the latter, say the political professionals, who have begun to recover their equilibrium after being discombobulated for a week by the most startling second political coming in many years.

"The polls {ranking Hart as an instant front-runner} represent a short-term fascination that won't stand up once people realize they are voting for president," said Frank Greer, a Democratic media consultant.

"They're going to contribute to Hart's undoing," said Greg Schneiders, a consultant to one of Hart's rivals for the 1988 Democratic presidential nomination, former Arizona governor Bruce Babbitt. "Maybe 10 percent of his 30 percent is a pure 'againster' vote that's always out there. The rest is just name identification. Hart's problem is that someone else is going to win Iowa and that person is going to become the Gary Hart of 1988 -- the media phenomenon. Then either that person or {Massachusetts Gov. Michael S.} Dukakis, or both, will beat Hart in New Hampshire. And then he'll be gone."

While not everyone subscribes to this quick-kill scenario, a conventional wisdom has begun to form which holds that Hart's rebirth will be short-lived. That wisdom comes, to be sure, from the very political establishment that Hart attacks -- to the applause of his supporters. But with that caveat, here is the insiders' assessment of the factors working against Hart:

The polls. Hart's ability to attract a 30 percent support level is a curse dressed up as a blessing. The one immutable law of presidential politics is that candidates are captives of their expectations. If Hart gets less than 30 percent once the voting starts, it will suggest that the electorate had sober second thoughts (just as voters did about him in 1984). For a candidate whose new slogan is "Let the people decide," this would be devastating.

His prior status. Hart's instincts, as well as his current situation, push him in the direction of running a guerilla campaign -- but that's hard to do for a former front-runner. His one clear advantage -- a lead in early polls -- could still help attract money, political organization and endorsements, but these are precisely the dross Hart now wishes to be liberated from. In late December 1983, he was much better positioned for guerrilla warfare; he was a genuine long shot, with 3 percent in the polls. That enabled him to claim victory -- and be treated by the media as the victor -- when he won a modest 16 percent in the Iowa caucuses, coming in a distant second to former vice president Walter F. Mondale. The reborn Hart may not want to discuss the events that forced him out of the race last spring, but he can't repeal his polling history. Last May, he was at 47 percent in the national polls and 65 percent in Iowa. He was once the odds-on favorite to capture the nomination. That's the history -- along, of course, with the Donna Rice episode -- he is running against now.

The calendar. It, too, is Hart's enemy. Iowa, which holds the first caucuses in the nation on Feb. 8, is a state where organization matters. The other candidates have staffs of 50 to 100 in the state. Hart led last weekend's Des Moines Register poll, with 29 percent of the vote from likely Democratic caucus-goers, but he cannot possibly replicate that showing on caucus night without an organization. And he cannot put one together in the short time remaining -- virtually all the activists are spoken for. Moreover, the people who attend caucuses in Iowa are not disaffected outsiders of the kind who might be drawn to Hart's new crusade. They are solid citizens who will cast their votes in full view of their neighbors. So Hart will lose Iowa. The candidate who wins Iowa will become the "story," and Hart will begin to lose his grip on the one thing that sustains him: media (and public) fascination with rebirths, media-bashing and sex scandals..

A week later comes the New Hampshire primary, where Hart scored a stunning upset of Mondale in 1984, winning 37 percent of the vote (against Mondale's 28 percent). An NBC poll taken over the weekend in New Hampshire (after a few days of Johnny Carson-David Letterman jokes and a general trashing of Hart by columnists and commentators) showed Hart with 16 percent of the vote (to Dukakis' 43 percent). It also showed that 53 percent of the Democratic voters have an unfavorable opinion of Hart -- a figure that makes him unnominatable and unelectable.

His "send 'em a message" message. It will take him only so far. Hart's reentry speech was brilliantly crafted to stir anti-establishment, anti-press juices. But Hart will have trouble sustaining this fervor. Analogies to George C. Wallace's 1968 and 1972 presidential campaigns miss a key point. The 1960s were a time of massive social upheaval; there was plenty the voters wanted to send messages about. What message does Hart propose they send? That a candidate's private life should remain private? That mistakes of judgment should be forgiven? That character is about more than libido? He will get takers on all those points. But a national crusade? Not likely.

His ideas. They are no longer new and they have always been more compelling intellectually than politically. Hart has a deserved reputation as a serious thinker on matters of domestic and foreign policy. But he is also a shrewd enough political tactician to understand that the electorate is not pining to have a plebiscite next year on light aircraft carriers, individual training accounts or restructuring Third World debt. For Hart, "new ideas" have always been a character card, his way of telling the voters that, despite a rather thin legislative record in the Senate and no executive experience, he's as qualified to be president as anyone -- including "anyone who has governed this country in the last 200 years," as he immodestly put it in an interview on the "MacNeil/Lehrer Newshour" last week. Hart's problem, reflected by his high negatives in the polls, is that, in addition to brains, voters take probity, judgment, morality, credibility and steadiness into their evaluations of presidential character.

The competition. Hart's other problem on the ideas front is that, contrary to what he said last week, most of the other Democrats are also running on Hart's "new ideas" -- and some have even come up with a few thoughts of their own. The fact that there was no deep issue debate waiting to explode in the Democratic field was a boon to Hart eight months ago, when he was the genuine front-runner, the candidate with the first franchise on the issues. But now, the fact that his rivals are saying many of the same things Hart says undermines his premise for reentering. The rivals will not be gentle about pointing this out to him in the debates ahead. They have already begun. "Gary Hart has a superiority complex," said Jesse L. Jackson, "without the superiority."

His uncampaign. It is not yet clear if the Federal Election Commission will release the $1 million in matching funds Hart is due from the first phase of his campaign in time for spending in January or February, when he will need it. Even if the money were made available in time, creditors from his 1984 campaign may bring legal challenges to block him from spending that money.

It is also unclear whether Hart has the time and resources to file delegates in the half-dozen big states where the procedure is costly and cumbersome. In Pennsylvania, for example, lists of delegate candidates are due to be submitted to the state party by Jan. 5. All the other campaigns have been working on theirs for weeks. Hart does not yet have a coordinator in the state. Once the list is submitted, each of 116 delegate candidates must get 250 signatures to qualify for ballot access. All of this takes organization and money. The rules are equally tough in Illinois and New York.

The man and his motives. When you ask yourself how a reborn Hart campaign might reexpire, you're lead immediately back to the question: Why did he get back in? The most plausible explanation comes from Ray Strother, Hart's media consultant when he had one. "You get the impression he had to do it, or die a bitter old man wondering if he got out too early."

For all his reputation for dispassion and bloodlessness, Hart returns to the presidential trail as the lead character in a personal and family psycho-drama. His wife, Lee, has spoken movingly of the pain the family has endured these past seven months. Hart himself has talked in television interviews of the anger his children felt toward him for quitting the race. "It was just awful," he said of his sense that he had let his children down.

This has been a sympathetic spectacle to some; a maudlin, overwrought and self-indulgent display to others. But it has been emotionally riveting (and therefore good copy and good TV) all around -- especially to the main character himself. Last week, in the midst of an interview with Time magazine reporter Robert Ajemian, Hart began defending his personal morality -- and he broke down and cried. "I don't weep for myself," he said, finally, according to the account in Time. "I weep for this country."

Hart is seeking redemption on two levels: To restore his good name with the public and to prove to himself and his family that he is not a quitter. Just by getting back in, he has already accomplished the latter.

The former will be more difficult. If the primaries don't go well for Hart, he has hinted that short of winning the nomination, he would like a role in shaping the party platform. But the prospect of his going to the Democratic National Convention in Atlanta, a conclave of the very insiders he's declared war on, is far-fetched. More likely, if the votes aren't there for Hart in Iowa and New Hampshire, he'll follow the course he outlined last weekend in a "60 Minutes" interview.

"I'm gone," he said. "I'm out. This isn't a dog-in-the-manger operation. I know when I'm not wanted."

The pros have already made it clear that Hart is no longer wanted. They're betting, for all the reasons outlined above and notwithstanding the polls, that the real "establishment" in this country -- the voters -- will soon be sending Gary Hart the same message.

Paul Taylor covers politics for the Washington Post.