Because registration is allowed on caucus day, the caucuses could draw independent voters and even some Republicans who are turned off by Trump. Moderate Democrats are actively working to turn out those potential voters.

"We’ve been proactive engaging Republicans to participate in the caucus," the adviser said.

Buttigieg is also courting Republicans and independents, while sounding alarms about Sanders. His campaign issued a fundraising email on Saturday with a not-so-subtle subject line: “Bernie Sanders could be the nominee.”

“I think that the higher the turnout, the more people that are participating, it’s going to be better for us,” said a senior adviser with Buttigieg’s campaign. “We want new people, we want Republicans, we want independents. We encounter a lot of Trump voters and a lot of Republicans, who want to be involved. I think we’re going to have real crossover appeal.”

Buttigieg’s appeal to potential swing voters, the campaign argues, is evidenced by the geographic areas of the state where he has held events and drawn large crowds.

Still, another Buttigieg adviser acknowledged that the spoils of high turnout could be split fairly evenly among the current top tier of candidates.

“It’s not like 2016, where you have the most establishment running against the most anti-establishment candidate,” the adviser said.

The caucuses will be an enormous test for Warren and Buttigieg’s field operations, which have impressed Democratic activists in the state for months.

Sean Bagniewski, chairman of the Polk County Democrats, said Buttigieg’s campaign recently drew about 300 people to a training for precinct caucus captains, enough for multiple precinct captains at some caucus sites next week. Warren has assembled such a vast army that her campaign is helping Bagniewski fill caucus chair positions at its caucus sites, he said.

“The fact that they have bandwidth to be helping the county party means they’re pretty damn organized,” he said.

A lower turnout election is still possible, and the lack of a spike in Democratic voter registrations has caused some observers to temper expectations.

But two strategists familiar with campaigns in Iowa said multiple campaigns were urging supporters in at least some areas of the state not to register until the day of the caucuses — a ploy to prevent other campaigns from targeting them ahead of caucus day.

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Chris Adcock, chairwoman of Democratic Party in Page County in southwest Iowa, said that if turnout is low, “I think [the beneficiaries] would be Sanders and Biden, because they are known quantities.”

But Adcock, like most activists in Iowa, expects turnout to be high. She has added caucus sites in her county, and she said Klobuchar and Warren’s campaigns could be rewarded next week for months of “working really hard.”

Organizers of a caucus site in Ankeny, just north of Des Moines, said they are expecting 100 to 150 more people to caucus Monday than did in 2016. And further west in tiny Audubon County, Sarah Christensen, a caucus chair, said of high turnout projections, “That’s what they say.”

“The Iowa Democratic Party sent me 200 caucus cards, and we've requested more,” she said. “I don't know if we'll need them, but I get the sense that we might.”

Holly Otterbein contributed to this report.