MINOT, N.D. — Heidi Heitkamp, a Democratic Senate candidate, called Leonard Rademacher a few weeks ago looking for his vote, but Mr. Rademacher, a 74-year-old retiree, was feeling ill, so Ms. Heitkamp called him back.

“I said: ‘Heidi, save your breath. I’m voting for you,’ ” Mr. Rademacher recalled, marveling at her personal attention. “I don’t necessarily agree with her, but I trust her.”

Gary Volk backed Ms. Heitkamp, a former state attorney general, after she sat for four hours on a slab of concrete next to what was once his house, listening to his struggles to recover from catastrophic flooding last year. Larry Windus’s mind was made up by an encounter with her opponent, Representative Rick Berg, a Republican, that ended with the candidate turning his back on him.

“He’s not very personable,” said Mr. Windus, 55, a dishwasher at Charlie’s Main Street Cafe here.

Senate Republicans considered the state in their column when Senator Kent Conrad, a veteran Democrat, announced his retirement last year. But with shoe leather, calibrated attacks and likability — an intangible that goes far in North Dakota — Ms. Heitkamp has made this a real fight. Though North Dakota is deeply conservative and is on no one’s presidential map as a question mark, this race could be one of the biggest surprises of the 2012 contests. And, like all close races this year, it could help decide control of the Senate.