Anyone can make a prediction. I’m going to show my work:
Romney has said he wants someone similar, not different, from him and someone who won’t go rogue in so many words. In other words, no Palins. I don’t think Portman is well know enough to add anything, and hasn’t really done much.
So, I’m guessing it’s T-Paw, who backed out of the campaign about a year ago before he had to fling much poo at Mitt. He fits the bill of having a background, at least superficially, in politics like Romney and he won’t draw too many unflattering comparisons.