It comes down to this
Dean supporters think their candidate is the best capable of short-circuiting the Republican plan to stereotype the Democratic nominee.
Clark supporters think their candidate is the best capable of short-circuiting the Republican plan to stereotype the Democratic nominee.
The good thing in this is that both camps understand that the basic plan of the Republicans is to run against a stereotype, not against whoever actually gets nominated.
As long as the supporters of either candidate don't presume that there is anything about their guy that automatically immunizes them from such attacks then I think either one of them can win.
They just have different strategies for doing it and none of us can really say whether either strategy has a better chance of working.
Hell, it's possible that neither strategy will work.
As long as we don't insult each other for favoring one strategy over another than we shouldn't have problems re-uniting after the primaries are over. It's when we start labeling the followers of a different strategy as "idiots" that the Republicans begin to cackle with glee. That's what we have to watch out for.