I chose Portman off the top of my head because they desperately need Ohio. But after thinking more about it, I'm changing my mind (as if these predictions mean anything...but it's fun).
I think he might want a Washington 'outsider'. Which translates into...a Governor. Although Rubio and other current DC pols seem to be popular choices, I just don't feel it. There is no need to 'fire up' the Tea Party, they are the most motivated to vote for Romney of anyone. That, and (in Rubio's case) Florida may be in the Red bag already, but with that said it is by far (29 EV's) the most important swing state.
There are 29 Republican Governors. Some of which add nothing to the ticket in terms of persona or more importantly Electoral Votes, because they are very Red States.
Here are 10 Republican Governors, from States that could arguably be in play -
Rick Scott - Florida (29 EV's)
Tom Corbett - Pennsylvania (20 EV's)
John Kasich - Ohio (18 EV's)
Rick Snyder - Michigan (16 EV's)
Chris Christie - New Jersey (14 EV's)
Bob McDonnell - Virginia (13 EV's)
Mitch Daniels - Indiana (11 EV's)
Terry Branstad - Iowa (6 EV's)
Rick Sandoval - Nevada (6 EV's)
Susana Martinez - New Mexico (5 EV's)
I crossed off Kasich and Scott because of the perceived baggage and Snyder and Corbett because Michigan and Pennsylvania probably aren't going Red in 2012. Chris Christie is also in a state that likely isn't going Red, and I think that is most important. Plus, again, you don't need to fire up the 'Tea Party', you need to move Independents. Indiana is going Red, bye bye Mitch. So is Iowa. And Nevada has more Mormons than any place outside of Utah. I don't know what the deal with McConnell is...he isn't mentioned much, which probably means there is some perceived baggage there as well.
So, if he goes with someone in DC, I still think it's Portman over Rubio and all the rest.
But if he wants to stay outside of DC, he might go with
Susana Martinez.
5 EV's is more significant than it seems. And they are having big problems with women.
If Romney gets Florida (very plausible) and one other big state, maybe Ohio (plausible), and then adds NM (if she is a popular Governor), that just might win him the election.
So, to sum, I'd predict there are one of three paths.
1) Portman - do whatever he can to win Ohio, assuming Florida will fall in the column.
2) Martinez - think bigger (women, long term strategy) and smaller at the same time.
3) The 'John Edwards' lay-up. Unify/pacify the party. And in that event, you could choose from several people, Rubio would likely be the most obvious, followed by Christie, then Daniels and then...(gulp) maybe even Santorum. Although I doubt it.
I would eliminate #3, personally. John Kerry said his biggest regret was choosing John Edwards over Dick Gephart (even before it became public what a douche Edwards was). That surely didn't go unnoticed. Unifying the party is way overrated. People, nationally, don't vote for the bottom of the ticket. (See: Quayle - 1988) That's why the EV strategy makes the most sense to me. Although it definitely doesn't always work (Gore - 1992) if I am Romney, this is the way I go. All those pissed off people are voting for you anyway.
A lot will obviously be determined by how those Florida and Ohio polls are looking around June. I think the major damage to come, from the Left side (in terms of negative ads) - is all going to be about women. That makes the most sense for them to do. It's easily done and the female demographic is obviously HUGE. Plus, there is possible help by having the surname 'Martinez' on the ticket with the conservative (read: religious) Latinos, of which there are MANY.