In the finger pointing at the tail end and aftermath of the Romney "campaign" there was one accusation that really stood out to me. I'm not sure how many of you live in or are familiar with what was going on in Ohio during the campaign, but part of what did Romney in, as you can read anywhere, was that Obama went on the air early with the "let them fail" ad re: Romney's stances on the bailouts. Romney decided to never specifically counter that ad. Now the accusation that struck me was that Senator Portman was apparently lobbying Romney to come out with an immediate response and point out that he actually offered alternatives to the bailout other than "let them fail."
Maybe if he's actually on the ticket, Portman would have more of a pull with the decision makers and could get them to run more specific responses and attacks in Ohio instead of the general sort of "jobs, jobs, jobs" ads.
Portman has his drawbacks. He's dull, not a great campaigner, and is still pretty unknown throughout the state. Of course the same could be said for Paul Ryan. Portman actually won a senate seat by a wide margin and his favorability rating among those who do know him his pretty high. He's also a great debater. He'll never win the charisma contest, but he prepped Bush and Cheney in 2000 and 2004. Joe Biden might be more likable, but I guarantee you he wouldn't be grinning through the debate like he was with Ryan. Portman is also taking over the NRSC after Cornyn leaves based on the strength of his understanding of state-wide campaigns.
So, in my completely unprofessional opinion, Portman would be the best choice. 1) He has a better chance of getting Romney's campaign to stop playing defense and run specific ads. 2) He's from a swing state and has actually won a statewide race therein. 3) He'll likely win the vice-presidential debate which will help Romney maintain momentum after the first debate. 4) He's sufficiently vanilla and understated that he'll keep the focus of the campaign on the president by not drawing attention to himself.
Now, can he win the election for Romney? Probably not. While it's likely that he could save the Ohio campaign, Portman doesn't add anything outside of the state except maybe, maybe, eastern Pennsylvania or Michigan. Given the margin of Romney's loss in those states, though, I doubt very much that any difference Portman made would swing those states. He doesn't play out west or in Florida. Honestly, though, there weren't many vice-presidential prospects who would be able to swing even one state for Romney, let alone save the election. That's why I think Portman was, far and away, the best choice for VP; he doesn't save the election, but he makes Obama the first president to win without Ohio since Kennedy.