DES MOINES — She began her campaign with small-town round tables, not big-city rallies. She pleads for Iowans to caucus for her at the end of her events. And long after the Democratic Jefferson-Jackson dinner ended here late Saturday, Hillary Rodham Clinton and Bill Clinton were still shaking people’s hands, as if to send a signal that they had learned a painful lesson about not taking Iowa for granted.

Nearly eight years after Mrs. Clinton was humbled by a third-place finish in Iowa, she has gone to great lengths to demonstrate her commitment to winning the state that first propelled Barack Obama to the presidency. But in a campaign that seems to be testing every long-held assumption about the electorate, Mrs. Clinton is facing a stiff challenge from Senator Bernie Sanders of Vermont that is again showcasing her difficulties with liberal activists in a state where she and her husband do not have deep ties.

She lacks the enthusiasm that Mr. Sanders enjoys. If he is able to turn out his backers from the progressive wing of the party and augment them with liberal independents new to the caucus process, he could be a threat to defeat her. Even some of Mrs. Clinton’s supporters here are reluctant to project much confidence, believing that the populist fervor gripping the left makes it dangerous to dismiss Mr. Sanders as a mere protest candidate.

“I really don’t think we have a clue,” said Representative Dave Loebsack, an Iowa Democrat and Clinton backer. “This year, not just on the Republican side but over all, there’s so much instability among the electorate.”