It's happening again.

You can hear the swoons and sighs, see the flush of excitement in living rooms from Cambridge all the way to the Upper West Side. The Democratic contest is obviously far from decided. But the most interesting story so far has been the unexpected appeal of Bill Bradley.

He's "authentic." He campaigns for "big ideas." He seems to reject poll-saturated politics. But for all the seeming novelty of his approach, he's actually running in a well-worn groove. Candidates often summon an echo of earlier leaders, sometimes without even realizing it. Bradley is best understood as the latest heir to the style of Adlai Stevenson.

The same kinds of college-educated Democratic voters who were "madly for Adlai" are now "madly for Bradley." Stevenson, the party's nominee in the 1952 and 1956 campaigns, was the hero for a generation of doomed idealists. He also, not coincidentally, was someone who eventually was rejected by the electorate again and again. It's a cautionary tale for Bradley if he wants to expand his appeal.

Stevenson's legacy is not so much a set of policies as a pose. Surrounded by grasping politicians, here was someone who advanced by seeming not to want the office too much. In his acceptance speech to the 1952 convention in Chicago, he said to rapturous applause, " 'If this cup may not pass from me, except I drink it, Thy will be done.' That my heart has been troubled, that I have not sought this nomination, that I could not seek it in good conscience, that I would not seek it in honest self-appraisal, is not to say that I value it the less. Rather it is that I revere the office of the presidency of the United States."

The Illinois governor presented himself as an answer to what would now be called "Truman fatigue." Truman was not known then as a peppery truth-teller but as an indecisive leader presiding over a corrupt administration. Stevenson appealed by seeming unusually authentic, a quality captured in the famous photo of him campaigning with a hole in his shoe. He made a great effort to build the myth that he wrote his eloquent speeches without outside help. (John Kenneth Galbraith, in his recent memoirs, tells of being summoned to Springfield to write speeches for the candidate, only to be detained in his hotel room. The governor had just been grilled at a press conference about why so many eggheads were converging on Springfield.)

In truth, for a newly educated, aspiring postwar middle class, some of the Stevenson allure was simple snob appeal. The Party of the People seemed to be a bit unsure about the People themselves. "You are the thinking person's candidate," he reportedly was told. Stevenson reportedly responded, "Yes, but I need a majority." The broader electorate has proven less taken with this pose.

Like hemlines rising and falling, this fad has recurred throughout the years. Gene McCarthy in 1968 (who, in fact, had nominated Stevenson). John Anderson in 1980. Paul Tsongas in 1992. All were candidates who appealed because they seemed somehow more diffident than those around them.

Bill Bradley's surprising surge marks him as the Adlai Stevenson for the SUV set. You could hear the echoes at the joint appearance (not even debatably a debate) in Hanover, N.H., when Bradley sleepily told the audience, "I would hope you'd feel I would be your candidate. And if you do, I welcome you. I need your vote."

Thus far, for all his talk about "big ideas," the catchiest idea he has presented is himself. Even on campaign finance reform, an issue on which he cares deeply and has genuinely led, he gives off an air not so much of anger at special interests as distaste for politics as usual.

For now, this approach appears to have great appeal. Bradley is doing especially well among "new economy" Democrats, who work with technology and who are plugged into the emerging digital marketplace. This must be especially galling for Gore, given his strenuous efforts to till Silicon Valley. These will also be key swing voters in the general election.

But recent history suggests that this delicate variety of liberalism wilts out of the hothouse of the primaries. It has taken years for the Democratic Party, sometimes dragged by Bill Clinton, to reposition itself as a party of mainstream middle-class values. Only recently has the party again been seen as trustworthy on taxes and government. Bradley's continued opposition to welfare reform--a keystone of the new Democratic approach--would be hard to explain in a general election. Stevensonian idealism has been challenged over the years by another, different strain of liberalism--tougher, more populist and more firmly rooted in middle-class cultural values. Robert Kennedy, at his best, represented that hard-nosed approach. After traveling with Stevenson in 1956, he wrote witheringly, "Stevenson was just not a man of action at all." He quietly cast his ballot for Eisenhower.

Bradley should be careful. The poses he strikes in November 1999 could come back to haunt him in the spring of 2000--or in November.

The writer, former White House director of speechwriting under President Clinton, is a fellow at the Institute of Politics at the John F. Kennedy School of Government.