In addition, Iowans still aren’t sure — after more than a year watching candidates here — that any of them can beat Trump. Asked to describe the candidates’ chances of defeating the Republican president, pluralities of Democrats said Sanders, Biden, Warren and Buttigieg can “maybe” win the general election, according to the poll. Fewer than 40 percent of Democrats say any of those candidates “probably would win.”

“I think [Trump] has an excellent chance of being re-elected, no matter what,” said Tracy Freese, the Democratic Party chairwoman in Grundy County, where she introduced Sanders at a town hall over the weekend. “There are plenty of Democrats who are realistic.”

The darkening mood has not changed enthusiasm for the election itself — even if it is tinged with anguish. Crowds in recent days were still jamming into coffee shops, community centers and high school gymnasiums to cheer their candidates.

Yet even energetic caucus-goers can have frayed nerves. Some Democrats worry an escalating conflict with Iran could help Trump politically. And despite projections for record caucus turnout, some party officials remain unsure what to expect.