Clinton leads Sanders 42 percent to 40 percent, a significant narrowing of her nine-point lead last month as her own appeal has eroded, according to a Bloomberg Politics/Des Moines Register Iowa Poll released Thursday. A third contender, former Maryland Governor Martin O’Malley, is at 4 percent. While Clinton publicly shrugs off the polls—“I just don’t pay that much attention to them,” she said Wednesday on CBS—the tightening race is reflected in the campaign rhetoric: In recent days, both she and her daughter, Chelsea, have stepped up criticism of Sanders, a one-time Senate colleague of Clinton’s. Sanders, a 74-year-old socialist who caucuses with Senate Democrats, though he is registered as an independent, is returning the favor, suggesting that Clinton’s more aggressive tone is a sign that her campaign is in “serious trouble.”

Just weeks before votes will be cast, the race remains fluid: 40 percent of those surveyed say they either have not selected a first choice or may be persuaded to change their minds about whom to back in the nation’s first nominating contest.

The poll findings suggest challenges facing both candidates: For Sanders of Vermont, the question is whether he can persuade first-time caucus-goers, the young people, and the independents who like his views to become voters. For Clinton, a former first lady, U.S. senator, and secretary of state—and second-time presidential candidate—it’s all about whether the poise and polish accumulated during years of resume-building will prove a liability in a year when voters are more sympathetic to insurgents.