All the other potential candidates come with more flaws. Rubio, 40 years old and fresh from the Florida Legislature, is untested on the national stage; as a Cuban American, it's not clear how many Mexican American votes he'd sway in the Southwest, and he might not even guarantee a win in Florida. Christie is a favorite of campaign reporters because of his New Jersey penchant for blunt, undiplomatic remarks, but that might be enough to disqualify him right there. Ryan would please the tea party, but he would also shackle Romney to the Ryan budget, which proposes turning Medicare into a voucher plan, an issue on which the candidate has tried to retain a bit of wiggle room. McDonnell is popular in Virginia, but outside the state he's known mostly for promoting state-mandated vaginal examinations for women seeking abortions — an idea he abandoned after a nationwide outcry and not one the Romney campaign wants to spend much time on.
A business strategist from Harvard Business School and the Boston Consulting Group, where Romney started, would look at that field and say: Choose the one who best reinforces your agenda with the lowest degree of risk. If those are the criteria, the choice is easy: Rob Portman.