Recent polls have painted a pretty bad picture of how unified the Democrats are. The most recent polls say that 55% of Obama voters would not be satisfied with Clinton as the nominee, while 72% of Clinton voters would be unsatisfied with Obama. Those numbers are from Mississippi, a state whose large black population tends to produce divisive racial politics, but the numbers are high in any state you’d look at now.
This sounds like cause for alarm, but I suggest being calm. The question is really of the form “how unhappy will you be if x happens?” and all our evidence suggests that people are unreliable with regard to that sort of question. In general, people overestimate the subjective impact of a setback, and show surprising resilience. For one extreme and somewhat disturbing case, try Dan Moller’s paper summarizing the evidence that when we die, our spouses rapidly move on (the author goes on to discuss the philosophical question of what’s bad about that fact).
So these reports about how satisfied or disatissfied people will be with the nominee are the equivalent of declaring “if Bush wins, I’m moving to Canada.” You don’t say you’ll be disatisfied with Clinton/Obama because you love them, but you don’t necessarily mean much by it. Lower numbers are better, since they do indicate something about the polarization of the electorate, but the numbers will always look misleadingly high.