In 2017 they donned pink hats to march on Washington, registering their fury with Donald J. Trump by the hundreds of thousands.

Then they flipped the House from Republican control, won the presidency and secured a surprisingly strong showing in the 2022 midterm elections, galvanized by their conviction that Mr. Trump and his allies constituted a national emergency.

This year, anti-Trump voters are grappling with another powerful sentiment: exhaustion.

“Some folks are burned out on outrage,” said Rebecca Lee Funk, the Washington-based founder of the Outrage, a progressive activism group and a purveyor of resistance-era apparel. “People are tired. I think last election we were desperate to get Trump out of office, and folks were willing to rally around that singular call to action. And this election feels different.”

But for Democrats, the mission is similar: Now defending the White House, President Biden is trying to reassemble that sprawling anti-Trump coalition, casting the 2024 contest as another battle to save American democracy as Mr. Trump moves toward the Republican nomination.

Mr. Biden, however, has a lot of work to do. Interviews with nearly two dozen Democratic voters, activists and officials make clear his challenge in energizing Americans who are unenthusiastic about a likely 2020 rematch, are worried about his age, and, in some cases, are struggling to sustain the searing anger toward Mr. Trump that Democrats have relied on for nearly a decade.

“We’re kind of, like, crises-ed out,” said Shannon Caseber, 36, a security guard in Pittsburgh who called the prospect of a Trump-Biden rematch a “dumpster fire.” She added, “It’s crisis fatigue, for sure.”