One thing I have noted over 80 years of watching Political Antics, no matter who the Dems select as a standard bearer, they will all rally around that person. Not the Pubbies if they don’t get their person they want to take their ball and go home.
At the rate things are going, it could be a bit like 1860. Maybe the race will end in a photo-finish: 25% McCain, 25% Hillary, 20% Romney, 20% Obama, 5% Paul, 2% Kucinich, 2% Buchanan, and 1% Steven Colbert.
But I think it's more likely Hillary will manage to crush Obama, a lot of Democrats will be mad for a few weeks, and then they'll all come back to support her out of hatred for the Republican Party.