Observed at close range, however, the D.S.A. feels less like a class expression than a cultural one. Offline communities built around shared interests — the sort once found in bowling alleys, all-ages punk-rock venues, church basements or radical bookstores — are rare now, and D.S.A. chapters fill this role. They offer not just a different politics but a different model for what participating in politics means. Chapter operations are heavily D.I.Y. affairs, which adds a giddy unreality to their victories, as if a group of college friends started a record label in their dorm room and somehow ended up using it to unseat an eight-term congressman. “It becomes a thing that you do to meet your friends — and to make friends,” Shua Sanchez, a 36-year-old organizer with the Milwaukee chapter, told me.

Even Democratic strategists who are wary of the group’s growing influence in the party grant that they have figured out something that the mainstream of the party now struggles to organically produce. “They’re real civic engagement,” said Adam Jentleson, a former staff member of Harry Reid’s and John Fetterman’s and president of the Searchlight Institute, a new Democratic think tank that is trying to drag the party away from the cultural politics of the 2020 era. “We don’t have Elks Clubs anymore. We don’t have union halls anymore, really. But we have these 50 people in a room discussing Robert’s Rules of Order. We shouldn’t let them determine the direction of the party, but we shouldn’t chase them out of the tent.”

There are recent antecedents to this in left-liberal politics — and not just the 2016 Sanders campaign. More than one D.S.A.er confided to me that their formative experience with grass-roots politics was the conventionally liberal 2008 candidacy of Barack Obama, whose two million volunteers, organized through Obama for America, propelled his campaign from long shot to the White House. Many Obama volunteers felt they were building a progressive force bigger than his presidency and were disappointed when Obama’s advisers shunted the volunteers off into the Democratic National Committee as a highly managed cheerleading operation for his administration.

“I knocked doors for Obama!” Sanchez, the Milwaukee D.S.A. organizer, told me. “And they never asked me to do anything again after that.” Megan Romer, the D.S.A.’s other national co-chair, who phone-banked for Obama in 2008, said that for the D.S.A., the experience had been “instructive”: “Like, how did that get dissolved? How did that get co-opted? Because there was organic energy.”

Renée Paradis, an election lawyer and a onetime progressive Democrat, also worked for Obama’s campaign, in 2008, and joined the D.S.A. immediately after Trump’s inauguration in 2017. “I was like, This elite theory of change is just absolutely inadequate to this problem,” she told me at the Brooklyn party.