"We may see a hundred-year storm for turnout"

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 in Council Bluffs, Iowa, where Mr. Sanders appeared at the first of three rallies in the state. “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.

The populist message many of the candidates have on offer is resonating: From Northern California to Council Bluffs to the Brooklyn streets where Mr. Sanders was raised, voters are delighting in the calls to spurn big donors, the policies to fight wealth inequality and the promises of relief from college debt and steep medical bills.

Ms. Harris kicked off her campaign in January with a rally in downtown Oakland before more than 20,000 people. Hundreds turned out to see Senator Kirsten Gillibrand last month at Dartmouth College, her alma mater. Even Senator Sherrod Brown of Ohio, who briefly toyed with running for president before bowing out this week, lured a throng of New Hampshire voters to a recent meet-and-greet at a bookstore.

But it is Mr. Sanders’s ability to muster supporters, and his focus at this point on big, showy rallies rather than smaller events like town halls, that perhaps best captures the early 2020 ebullience. At this stage of the race, his events are also doubling as shows of force — supporters filled a Navy Pier hall in Chicago — evoking the strategy of President Trump, whose 2016 campaign gathered momentum in part because of the large rallies he held before similarly boisterous crowds.

Some 2,000 people, many from neighboring Omaha, came to see Mr. Sanders in Council Bluffs on Thursday, erupting more than once into now-familiar chants of “BER-nie! BER-nie!” As a band played tunes in an exhibition hall that smelled of freshly popped popcorn, small children amused themselves on a makeshift bed of winter jackets as a group of adults looked on.

On Friday night, an overflow crowd of more than 1,300 turned out to see the Vermont senator in Iowa City.

Early polling data underscores the displays of grass-roots enthusiasm: A recent University of New Hampshire survey showed that more than 60 percent of Democrats said they were “extremely interested” in the primary, significantly higher than they reported at this stage in each of the last three cycles. Overall, half of those polled in the state said they were “extremely interested” in the primary.

While the energy has been uplifting for many of the candidates, it has also posed something of a challenge for anyone looking to gauge early-stage popularity: Because voters are showing up in such high numbers and cheering so enthusiastically, even for lesser known candidates, the traditional measures of excitement — crowd size, noise — no longer distinguish individual contenders. (Two candidates who have yet to join the race, Beto O’Rourke and former Vice President Joseph R. Biden, are expected to bring big crowds in their own right.)

“In past cycles, you were there for Edwards, you were there for Barack Obama, you were there for Hillary, you were there for Bernie,” said Sean Bagniewski, the chair of the Polk County Democratic Party in Iowa, which includes Des Moines. “All of our Democrats take the prospect of defeating Donald Trump so seriously that it’s almost like everybody is on the same team.”

The early grass-roots enthusiasm is also buoying email lists and fund-raising numbers. Ms. Harris’s campaign, for instance, boasted that it had raised $1.5 million in its first 24 hours. And Mr. Sanders’s campaign said it had collected $10 million from 359,914 donors in its first week, an extraordinary number that underscores the power of his small-dollar donor base. His campaign also says that more than one million people have signed up as volunteers.

Although many people have not yet decided which candidate they will support, political watchers say the level of engagement is comparable to what they typically see much closer to the primaries and portends high voter turnout.

Michael McDonald, an associate professor of political science at the University of Florida who studies voting data, said he believed enthusiasm and voter turnout were correlated.

“We already know interest is running high given so many other indicators, so I expect turnout will run high for the 2020 primaries on the Democratic side,” he said in an email. In the general election, he added, “We may see a hundred-year storm for turnout.”