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A lot of people are excited about Obama's run, and a lot of people just aren't excited about Clinton's run (putting it politely as I can). That makes all of the difference. Clinton only barely beat "Uncommitted" in Michigan. That should be a big warning for the candidate.
Given the relative similarity between the candidates' stances (and, let's be honest, they are both distressingly center-right in their stances, which passes for leftism in America's woefully right-skewed political continuum), the politics of personality come into it in a big way -- that is, because Obama and Clinton are pretty close overall in their views, it comes down to which one more people like better (sorry, Edwards supporters -- your guy's finished), and more people like Obama than Clinton.
Obama brings in more people, and Clinton repels more people. It doesn't have to do with black/white and man/woman; it's just an issue of charisma -- Clinton's smart, but lacks Obama's charisma. Obama's smart, and has charisma to spare, so he's the better candidate in the personality game. The Clintonites resent the hell out of that -- we're just supposed to be so dazzled by Clinton's brains that we're supposed to ignore that she's just not charismatic, that she's uninspiring -- the John Kerry Effect. Obama will bring more people into the Democratic Party, and HR Clinton will drive them away (and energize the GOP against her) -- so, Obama's a winner, Clinton's a loser, at least along the lines of charisma and personality.
Charisma does matter in politics -- Bill Clinton would never have made it if he didn't have charisma to spare; he pissed me off with his political gutting of the Democratic Party, and I still like the guy. The trouble is, Bill can't teach Hillary charisma; you either have it or you don't. And she doesn't.