It's not her honking laugh, political baggage or pantsuits.
Sen. Hillary Rodham Clinton lost the Democratic nomination for president because of "gender bias" in the news media, according to a Johns Hopkins University analysis that found the New York Democrat has attracted marked press discrimination.
But so has the rest of a rare sorority - eight other women who have run for president over the decades. The research also found that journalists have favored male politicians over their female counterparts for 136 years.
"Ninety-two percent of Americans say they would vote in a presidential election for a qualified female candidate from their own party," said Erika Falk, a communications professor who led the study.
"Asked if America is ready for a woman president, 55 percent said, 'yes,' a figure that has been steadily rising since the middle of the 20th century. So where is our woman in the White House?" she asked, citing recent figures from CBS News and New York Times polls.
Along with Mrs. Clinton's current treatment in the press, Ms. Falk has analyzed the news coverage of an octet of female presidential hopefuls, beginning with Victoria Woodhull in 1872 and ending with Carol Moseley Braun in 2004.
"The most striking finding was that women consistently got less coverage than male candidates in the same race, and that did not improve over time," she said.
The gents, in fact, got twice as many stories and their stories were 7 percent longer - a trend Ms. Falk said could ultimately dampen the political aspirations of women fearful they would be "held up for ridicule and scrutiny."
The tone of the stories also came into play - along with stereotyping. Male candidates received much more "substantive coverage," with 27 percent of it focusing on their policies and issues, compared with 16 percent of the stories about the women.
"Stories about female candidates emphasized their physical appearance and families. For the women, there were three times as many physical descriptions - references to clothing or age - as their closest male competitor. Women were also stereotypically portrayed as more emotional and their professional titles were more likely to be omitted from stories," the study said.
Similar forces are at work in the 2008 campaign, with Sen. Barack Obama of Illinois garnering more prominent news coverage than Mrs. Clinton. When both candidates announced their intention to run in 2007, for example, Mr. Obama was the subject of 59 major newspaper stories, compared with 36 for his rival, Ms. Falk said.
Other research echoed the phenomenon.
Far more Americans believe that the press coverage has favored Barack Obama than they think it has favored Hillary Clinton. Nearly four in ten (37 percent) say that in covering the Democratic race, news organizations have been biased toward Obama while just 8 percent say they have been biased toward Clinton," said a study released Thursday by the Project for Excellence in Journalism.
Ms. Falk has compiled much of her findings in the new book "Women for President," released in January. She also advises future "Madame Presidents" to get tough and plan accordingly.
"They have to tailor the strategies to overcome this bias," she said.