One big thing to consider when mulling over possible VP selections for either party is this: If the candidate is a sitting congressman and is elected VP, who's going to replace him? With razor thin Democratic majorities in both houses, neither party is going to want a VP candidate whose seat might be given to the other party by gubernatorial appointment or special election. For example, there's absolutely no possibility that Obama is going to select Bill Nelson of Florida, because Florida's Republican governor, Charlie Crist, would get to appoint a replacement for at least the next two years of Nelson's term. That Joe Biden isn't afflicted by a similar problem (Delaware's governor is a Democrat) is a point in his favor.

On the other hand, McCain/Lieberman makes a lot of sense vis-a-vis any Republican attempt to regain control of the Senate. Though technically an independent, Lieberman is counted as a Democrat. If he were to be elected VP, either Connecticut's Republican governor would get to appoint a Republican replacement or a new senator would be elected in a special election, depending upon the provisions of that state's constitution.