They are, at least, hastening the tectonic shifts taking place in the party. It was no accident that House Democrats modified a resolution targeting Representative Ilhan Omar of Minnesota — for her controversial claim that pro-Israel advocates carry an “allegiance” to a foreign country — after Ms. Ocasio-Cortez, other lawmakers of color and the party’s leading presidential hopefuls rebelled against singling out Ms. Omar. The episode marked a striking departure from the down-the-line support for Israel that has characterized the upper ranks of most Democratic primaries.

Relying on wealthy donors has also been standard fare, and unremarkably so, in past White House races. But Mr. Sanders elevated the issue of campaign finance in his 2016 race, and his jeremiads against the billionaire class have all but made super PACs verboten in this race; he and Ms. Warren are refusing to hold high-dollar fund-raisers.

Representative Ayanna Pressley of Massachusetts, who won her seat after defeating an incumbent Democrat last year, conceded that the first months of 2019 had inflicted “growing pains” on Democrats, but she argued it was merely a necessary side effect in the party’s evolution.

“After we build that muscle, we’re going to flex it,” Ms. Pressley said.

Yet Mr. Biden, in speeches at home and abroad, has used much of the first part of this year pledging to restore the dignity he believes that the country has lost in the Trump years, promising a restoration rather than a revolution. And, as his supporters put it less subtly, his campaign would represent something else.

“Overwhelmingly, the primary electorate of the Democratic Party wants to win,” Senator Chris Coons of Delaware said. He argued that Mr. Biden could “repair a lot of the ways in which our position in the world has been harmed” while offering a “hopeful, optimistic, positive” vision at home that would heal the divisions he said Mr. Trump has exacerbated.

To such moderate Democrats, the most instructive recent election is not that of Mr. Trump in 2016 but rather the 2018 midterms, when many of the Democrats who won in battleground House districts and governor’s races were decidedly less confrontational than Mr. Sanders.

“The overwhelming majority of seats we picked up were by center-left candidates representing more centrist-type districts,” said Representative Brendan Boyle of Pennsylvania, adding, “There’s still lots of folks on our side who are O.K. with compromise.”