Called the Democratic Renaissance Project, it attracted a rotating cast of ambitious 20-somethings looking to get past the doldrums of the Bush-Clinton-Bush era without succumbing to utopian radicalism. As these elder millennials aged into maturity, the same ideal gave rise to new journalistic outlets like Vox. They gained new champions too, like the Warren protégée Katie Porter, who turned her suburban Orange County, Calif., congressional district blue in the 2018 wave election. By the time the 2020 campaign got underway, liberal reformers had a bevy of candidates to select from, none more impressive than Ms. Warren.

Except here we are, refighting 2016, with Joe Biden taking up Mrs. Clinton’s role and Bernie Sanders playing himself. To baffled Warren supporters, the last year seems like a string of avoidable disasters. If only activists hadn’t been so fixated on Medicare for All, then she could have finessed her way around health care. If only Alexandria Ocasio-Cortez hadn’t endorsed Mr. Sanders after his heart attack, then he could have quietly wrapped up his candidacy. If only the Sanders campaign wasn’t a personality cult that refused to take yes for an answer, the left could have united behind a politician with a real shot at winning.

But that misses the true nature of the movement that has grown up around Mr. Sanders. Ms. Warren tried to bend the Democratic Party to the left. Mr. Sanders’s core supporters are intent on remaking it from the ground up.

They want a new coalition grounded in the multiracial working-class and less dependent on affluent professionals; a new donor class made up of grass-roots contributors; a new base of activists who read magazines like Jacobin and come out of groups like Democratic Socialists of America; and new politicians like Ms. Ocasio-Cortez, who trounced New York’s Democratic machine.

Mr. Sanders’s most loyal followers are as much part of a counterculture as they are members of a political campaign. Rather than asking the best and brightest to lead the way beyond left and right, they have come up with a novel fusion of populism and socialism that marries a critique of the inequalities generated by capitalism with a rejection of technocratic nudging and meritocratic striving. Tell them that Elizabeth Warren is the real radical, and they’ll ask what you can expect from an administration dominated by products of the same elite institutions that ran the Obama White House. Insist that they should be practical, and they’ll wonder how progressives will be able to change the country if they can’t even change the Democratic Party. See the world from this perspective, and Ms. Warren looks like the left wing of a broken status quo, not the start of something different.

Yes, the Sanders campaign has its fair share of Ivy-trained policy specialists. But to its millennial base, the difference between their tribe and the rest of the party is obvious at first sight. It’s what separates Ms. Ocasio-Cortez from Katie Porter, Jacobin from Vox and Democratic Socialists of America from the Democratic Renaissance Project. They can’t stand MSNBC; their attitude toward Russia, Ukraine and impeachment tended toward indifference; and don’t get them started on “The West Wing.” While Mr. Sanders offered them red meat, the other candidates were trying to sell an Impossible Burger.

The problem for Mr. Sanders is that this group is still a distinct minority among Democrats, and the populist revolution that was supposed to sweep new voters to the polls has failed to arrive. But Democratic leaders shouldn’t celebrate for long. Mr. Sanders remains a formidable opponent, and President Trump will be waiting in the fall. The Democratic establishment has put all its chips on Mr. Biden, and the costs will be high if the gamble doesn’t pay off.