I would think that a voter would want to take into account who could win in a national election. When you look at Obama's issues and compare them to Hillary's issues, they agree on more than they disagree. I would argue this election is more about the cult of personality than anything, and Obama has that charisma that could bring many independents and some Republicans to his side, especially considering in how much turmoil the Republicans are in trying to find an acceptable nominee.
Hillary Clinton is disliked by 49% of the population. That may hurt her feelings, but she does represent old-school, old-style politics, the kind that turns a lot of people off. I do not see her bringing more people in under the tent unless the GOP nominee screws up so badly, moderate and conservative voters say to themselves, "Well, at least it'll make history to have a woman be president."
Since 1980, there has always been a Bush or Clinton on the ticket. We have a two-family monarchy in US politics right now. I'm 34, and I barely remember Pres. Carter. Pretty much my whole memory is having one of the Bushclintons in office, and if we're going to look forward to the future, a clean break needs to be made. I think that's part of Obama's appeal.
A lot of Americans would like to have that historic moment when a woman becomes president. But most of them don't want it to be someone as calculating and Machiavellian as Hillary. So there's another historic moment they'd like, when an ethnic minority becomes president, and there's Barack Obama. Sunny, inspiring, seems honest, has a knack for communicating, someone who could actually unite the country. Yeah, he has a liberal record, and I'm a Republican, but I know if Obama gets the Democratic nomination, odds are I won't mind either candidate winning the general election (what a dream that would be), and if the wrong guy gets the GOP nomination against Obama, for the first time in my life, I'll vote for the Democrat for President.