Campaigns battle cold and complacency in latest Iowa turnout

Trump also benefits from fiercely loyal fans, some of whom have driven many miles to support him without having had any contact with the campaign.

Edward Michaels, a 66-year-old truck driver, said he was driving from his home in Dallas, Texas, to volunteer. “We don't want to take any chances,” he said in an interview Thursday at the Machine Shed restaurant in Urbandale, where the former president's eldest son, Donald Trump Jr., was gathering supporters.

Amid all this uncertainty, Ms. Haley is the biggest wild card.

Until late November, she had virtually no campaign staff on the ground in Iowa. But then the political network, founded by the billionaire industrialist brothers Charles and David Koch, supported her and transferred their organizational power and financial strength. The group, Americans for Prosperity Action, is knocking on doors across Iowa for Ms. Haley, trying to reassemble the coalition of wealthier, college-educated Republicans that brought Senator Marco Rubio of Florida close to victory in a quick finish in the caucuses in Iowa in 2016. .

The weather can play to that group's advantage. Those voters are concentrated in urban areas, where the roads have been cleared and it will be easier for them to reach an election rally. It is unclear which factor will be more important in her battle with Mr. DeSantis for second place.

“If you were to stick a gun in my head right now, I would probably rather be Nikki Haley because she has had the better trajectory over time than DeSantis,” said Mr. Kochel, the political strategist, referring to the fight for second place. “But boy, I wouldn't bet a mortgage payment on it right now.”

Reporting was contributed by Jasmine Ulloa, Reid J. Epstein, Kellen Browning And Lisa Lerer.