“I’ll still vote,” said Becci West, who manages a pizzeria in Marshalltown and has a 13-month-old daughter. She was one of the last mothers to deliver her baby before the local hospital shut down its obstetrics ward. “Health care is important to me.”

Four years ago, Ms. West said, she voted Libertarian when Marshall County swung by 18 points into Mr. Trump’s column. Two years after that, in the midterms, the county was part of a Democratic surge that unseated a two-term Republican congressman and elected Abby Finkenauer, a Democrat who stressed her rural roots and family’s union ties. Now, like so many counties spreading east from Des Moines to the Mississippi River, it is a jump ball.

Democrats across Iowa said they worried the caucus turmoil would amplify the problems of a disappointing turnout. The caucuses were attended by 176,000 people, about 3 percent more than those who showed up in 2016, and far less than the 300,000 some campaigns had prepared for.

In some rural precincts, volunteers were distressed that only 15 or 20 people showed up to school gyms and civic-center basements where twice that many had come in past years.