Recent polls have shaken the core of the Democratic race. For the first time in months, someone not named Clinton leads in Iowa, and that same someone -- Barack Obama -- is rapidly catching up in New Hampshire. Hillary Clinton, the once inevitable lock for the nomination, suddenly looks vulnerable. But other polls out this week show Clinton still leading, albeit by slim margins, among Iowa caucus-goers.

The difference between the two divergent sets of results can be found in the cross-tabs and holds the key to Clinton's success. When she leads, Clinton enjoys a large gender gap. When she trails, Obama closes that gap among women, effectively shutting off Clinton's most valuable base.

Consider some recent polls. In a Pew Research survey, Clinton enjoyed an eight point edge over Obama among women in Iowa, a 29-point gap in New Hampshire and a 17-point lead in South Carolina. Consequently, Clinton led in all three states.

But in a recent Des Moines Register poll, in which Obama led among Iowa caucus-goers, Clinton's advantage among women had evaporated. Obama attracted more women to his side, 31%, than Clinton, who was favored by 26%. Compare that to Clinton's 34%-21% lead in the Register's October poll. A Washington Post/ABC News [pdf] poll, the first to show Obama leading in Iowa, was also the first to show Obama leading among women -- 32% to 31%.

Pollster Ann Selzer, who conducts the Register's polls, says women react to Clinton and Obama very differently. "There's a real difference between the candidates in distinguishing their leadership style," she said. Those who have questioned Clinton recently on her records at her husband's presidential library have cemented an opinion that she stands for the secrecy and triangulation of the 1990s. "People harken back to a time when [they] felt like things weren't on the up and up," Selzer said. Obama, though, brings a perceived openness to the race. By stressing his ability to solve problems through compromise, he is naturally appealing to women's styles.

Clinton needs women to win the Democratic nomination, and she needs women to win a general election. Recently, the candidate has been showing off her motherly, feminine side in order to boost her credentials with women. But as more women choose a candidate not named Clinton, some on her team have to be wondering how they put together the coalition necessary to get a victory.