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INDIANOLA, Iowa — A few hours before his first solo run as the chairman of an Iowa presidential caucus on Monday, Chris Nelson was headed to the dentist. He had placed strict restrictions on the appointment. “I don’t think I can pull any teeth today because I don’t want to sit up there with a drooling kind of look,” Mr. Nelson, 53, said, with a laugh.

In 2012, when he was a co-chairman, only six voters showed up to Mr. Nelson’s precinct in this city of 15,000, a bellwether spot where each of the three Democratic presidential candidates has visited at least twice. This year, with a competitive race, Mr. Nelson, who builds vending machines at a nearby factory, was expecting up to 100.

His caucus, held in the western wing of a bright cafeteria at Indianola High School, was beginning to fill up when he arrived a few minutes before 6 p.m. Asked if his caucus would involve less conflict than a Republican one, Mr. Nelson, a supporter of Senator Bernie Sanders of Vermont, laughed for a full five seconds. “I just hope everything goes smoothly,” he said.