Jonathan Bernstein points to a hole in our understanding:
In my view, political scientists haven't done a very good job at all of sorting through the concept of ambition. The literature talks about ambition for office, which is fine as far as it goes (and is basically what I used in my post the other day), but I don't think it really goes very far. Think, for example, about Dick Cheney.
It's true that as Veep he didn't have "progressive ambition" -- he didn't run for president, and as far as we know really didn't have any intention of running for president. And as Seth says, that had real consequences. Yet I think it would be a real mistake to say that he had no ambition at all; it's just that his ambitions were devoted (apparently) towards amassing influence during the Bush presidency, not towards having a presidency of his own. Overall, I just think we collectively don't quite have a handle on ambition.