“I don’t know if Trump runs in 2024, but I’ll tell you one thing: If someone comes along who has enough intelligence to understand what Trump did, they’re going to be an extremely tough person to beat,” said Dan Smicker, the chair of the Republican Party of Clinton County, Iowa.

Democrats had won Clinton County, which is north of Davenport, in the seven presidential elections before 2016, when Mr. Trump flipped it. This year, the president more than doubled his winning margin there, to 10.3 percentage points. Just eight years ago, Mr. Obama carried the county by nearly 23 points. In fact, Mr. Biden did not win back a single one of Iowa’s 31 pivot counties, the most of any state.

Mr. Smicker, a former high school agriculture teacher, said Mr. Trump had increased his support in rural towns where farmers embraced his trade wars — and even more so in the working-class county seat of Clinton. “If you’ve seen a drain of manufacturing jobs and all of a sudden you have someone who shows a lot of interest in redeveloping that, people will vote their interests,” he said.