And as the Democratic National Committee moves to eliminate superdelegates — the elected officials and party elites who help determine presidential nominees — there is widespread expectation that traditional power brokers should cede more authority to the activists on social media, often millennials and people of color, who are increasingly steering the party’s agenda.

“We have to pay attention to our base,” said Representative Pramila Jayapal of Washington, who was the only lawmaker arrested at the immigration protest last week. Pointing to a series of special elections and primaries, including Mr. Crowley’s, that she said showed a rising liberal coalition flexing its power, Ms. Jayapal added, “That energy, combined with the real threat of a second Supreme Court justice that could strip away women’s reproductive rights and a lot of other rights that people have come to rely on — I think it is an even bigger call to action.”

The turmoil on the left mirrors that of Republicans in the first two years of Mr. Obama’s administration, when Democrats controlled all the levers of government and left the Tea Party-inflected Republican Party to thrash around in impotent protest, raging with an energy that eventually propelled it back to power.

But some Democrats see the moment in even more sweeping terms, akin to the era following the Vietnam War and Watergate, when the reaction to a controversial Republican president triggered a moderate and liberal backlash. That movement delivered dozens of new seats, but it also unleashed a generational changing of the guard that jolted party leaders.

Former Senator Gary Hart of Colorado, elected in the Democratic wave of 1974 and a leader in the effort to reinvent the Democratic Party in the 1980s, said Democrats were approaching an overdue moment of reckoning with their own limitations. The party, he said, had failed for years to define a forward-looking vision. The pressure of the midterm election — heightened, he said, by the Supreme Court vacancy — could create a new moment of definition.

“There almost has to be a generational renewal of belief systems,” said Mr. Hart, a two-time presidential candidate, who is now 81. “When we were in power, under Obama and Clinton, I don’t believe party leaders did what should have been done, and that is come up with a manifesto for the 21st century.”