I agree with a Portman pick, just because he would make an excellent VP and strategically he helps in Ohio, a crucial state. I get the contrast stuff brought up above, but I'm a little bemused by the interest in Pawlenty. The party establishment clearly dislikes him, and the primary voters proved to be quite underwhelmed with him too. Plus, even though Biden is a gaffe machine, I think Pawlenty would look bad on a debate stage next to Biden.
Polls show that Portman is not really all that useful in Ohio, actually.
That might be true now, but would Pawlenty help in Ohio, Virginia, or Florida? Portman has some baggage, but he's a better debater and a respected government veteran.