Who's Hart?

Gary Hart's flash to co-front-runnership for the Democratic nomination has been meteoric, truly a political phenomenon. He came from second in Iowa's caucuses to victories in New Hampshire and Maine and to an expected win in Vermont's nonbinding primary today. He is leading in the Massachusetts primary, looking for a clean New England sweep. In national polls, he's pulled from 2 to 1 behind Walter Mondale to even in a week.

What's going on?

It takes nothing away from Mr. Hart to point out that this blinding speed in his rise raises the prospect that the Democrats could quickly commit themselves to his candidacy without knowing much about him at all.

Next week is Super Tuesday, with nine states picking convention delegates. The Democrats who designed this year's nomination race course deliberately front-loaded it, bunching events so that a front-runner - namely Mondale with labor's endorsement - could wrap it up early.

A flaw in the system all along was that an outsider might shock the front-runner, catch a wave of publicity and fervor, and benefit from the very same front-loaded system designed to confirm the party establishment's choice. Americans in both parties show a strong streak of populism; they don't always want to listen to what the party bosses say.

In any event, Hart is suddenly enjoying incredible press attention. The number of newsmen and cameramen traveling with him has increased tenfold. In the crucial weeks ahead it's going to take some sober detachment to look at his candidacy in terms of how much is media surge and how much is substance.

''It's going to be possible for Hart to make a killing on Super Tuesday without anyone knowing what he stands for, who he is, and where he comes from,'' warns Thomas Mann, executive director of the American Political Science Association and an adviser to the Democrats on nomination reform. ''As a political quantity, to most American voters Hart is only a week or two old.''

This Sunday night, the League of Women Voters is sponsoring a Democratic candidate debate in Atlanta. That's the time, on the eve of the next big round of primaries, to ask Hart directly to spell out what has been his rather amorphous ''new ideas'' claim. What about the arms control variations - build-down, freeze? What about budget deficits: How would he treat social spending, taxes, defense? What about Central America, the Middle East?

Hart's surge is striking partly because Walter Mondale has made no particular mistake. Mondale still has the support of the Democratic establishment. Many political analysts still expect Mondale to prevail. Mondale's prospects appear stronger in identifying with working-class, traditional Democrats, while Hart's appeal seems to be settling among upwardly mobile, middle-class Democrats and independents. It still could be quite a race. John Glenn, Jesse Jackson, and George McGovern still have their laps to run.

Democratic voters in 1980 whipsawed Jimmy Carter and Edward Kennedy week to week, defecting to Kennedy when the not-so-popular Carter seemed a certain winner, abandoning Kennedy if there seemed a threat Kennedy might win. In 1984, there has been no passion about Mondale's campaign. He's been seen as able and intelligent. This has not been enough for voters to accept him as ''inevitable.'' Hart, with his generational, new-ideas appeal, may prove the counterpoint to Mondale's stolidity. Or he may be the candidate the voters might want. A closer look at Hart would help.