They dragged their friends to see Bernie Sanders in Iowa. In South Carolina, their unrelenting selfie requests made Cory Booker late. And in New Hampshire, so many showed up at a church for an event with Kamala Harris that an overflow crowd had to stand outside in the snow.

“I’m super overwhelmed by the number of Democratic candidates that have already come out,” Regan Johnson, a 28-year-old from Omaha, said on Thursday before Mr. Sanders’s rally here in Council Bluffs. “Hopefully the excitement continues and we’re able to get a good, viable candidate that can beat Trump.”

As the already large presidential field grows by the week, the enthusiasm that propelled Democrats to a decisive takeover of the House in the midterms is still surging, driving crowd sizes and intensity typically seen in the days before the first caucuses and primaries, not a year ahead of them. Powered by an almost desperate yearning to oust President Trump, and galvanized by the most diverse field in presidential primary history, Democrats are packing into gymnasiums, churches and exhibition halls to hear candidates speak — even if they are far from committed to supporting the candidate they are showing up to see.