I think Portman is his best option since there's no realistic way for Romney to win without capturing Florida and Ohio. Since Ohio is a harder pull for Mitt (he's far more likely to win Florida and not Ohio, rather than the other way around), it would make sense to get all the help he can in that state.
McCain states + Iowa + North Carolina + Indiana = 212 EVs currently for Romney
+Florida and Ohio = 259 EVs
I really doubt he'll go with some inexperienced senator from a state he would still have only a fair chance of pulling and nabbing a measly 4 EVs.
Despite the increase in electoral votes to many red states from 2008 to 2012, the advantage in the electoral college is still with the Democrats.
The states John Kerry won in 2004 have gone blue in every election since 1992. That is, the last time any of those states went red was back in 1988. So those 18 states are safe for Obama. I would consider them solid or hard Blue. Next are 3 states I would consider moderate Blue, like Iowa, New Hampshire, and New Mexico. Those states have gone blue every time since 1992 except ONCE. I'd put them in Obama's pocket as well.
With those 21 states, Obama has 257 electoral votes. He is only 13 away from victory thanks to dark blue or moderately blue states. His easiest toss up state to win is Virginia because the economy has been great there the past four years unlike the rest of the country.
In fact, the only way the GOP can win Virginia is if they choose Bob McConnell the popular Governer of Virginia, to be Romney's running mate. Without him, there is no way that Romney can win the election, because there won't be any way for him to win Virginia without Bob.
Ohio and Florida are vital and if the GOP didn't have a chance of winning them, that may as well not put up a candidate for President.
I think the reason Obama has been able to to be so successful politically in difficult economic times that would sink most Presidencies is because of the extreme division in the country that has developed over the past 25 years based on party and geographical lines. It literally is the worst its been since the Civil War, although not to imply the country is headed for a civil war since that is literally impossible. It just means that you have two sides that are large and bitterly divided over politics which means no national politician is ever totally or totally in terms of their chances of winning. There always sitting on top of a nice large loyal base that provides a cushion which prevents them from falling into non-competitive territory for the election.
So thats why I think were about to re-elect a President with 8.2% unemployment for the first time since World War II.
Also, I think there are only 5 states that are in play that can decide the election. The rest are definitely Blue or Red and won't change.
The only 5 states where the votes will really matter this year are Virginia, Ohio, Florida, Colorado, and Nevada.