Mr. Forbes, a pharmacist, attended a debate-watch party at a Mexican restaurant in Urbandale, where the shuttered former headquarters of Gov. Scott Walker of Wisconsin, who quit the Republican race, was a few doors down. Before the debate, he said the big issue for Mrs. Clinton was “character and trust” because of her use of a private email system as secretary of state.

Mr. Forbes said he did not think she laid the questions to rest. But he was in a minority: Most debate-watchers said the highlight of the night was Mr. Sanders’s saying “the American people are sick and tired” of the issue of Mrs. Clinton’s “damn emails.”

Mrs. Clinton satisfied questions about trustworthiness when she noted that President Obama had trusted her enough to pick her to run the State Department, said Becky Schmitz, a social worker in Jefferson County. Before the debate, she leaned toward Mrs. Clinton but had doubts. Afterward, the doubts were gone. “She strengthened my position towards her,” Ms. Schmitz said. “She was very poised and strong, and I thought she carried the debate.”

That was a sentiment shared even by Democrats who did not come away supporting Mrs. Clinton.

“I’d love to be a Hillary girl because I’d love to see a female president,” said Peggy Sherrets, the mayor of Oelwein, population 6,400, in northeast Iowa. “I’m just not sure she’s electable. I’m wondering if her time hasn’t passed.”