The New York Times's Nate Silver estimates that running mates provide a 2 to 3 percentage point boost in their home states, with the effect increased in smaller states and diminished in larger ones. That margin could prove the difference between a second term for Obama or a first for Romney, "but it is probably not enough to outweigh the other strengths and weaknesses that a vice presidential candidate could potentially impart," Silver wrote in April. But a recent study for the University of Virginia's Center for Politics by Joel Goldstein, author of a book on the vice presidency, found that in the last 50 years the frequency of running mates from big states has dropped dramatically. As politics has become more nationalized, vice presidential selection strategy has focused on harder-to-quantify strategies like image reinforcement (see Bill Clinton's 1992 selection of Al Gore, doubling down on his "new Democrat" theme).
One rule remains: First, do no harm. There are more No. 2s remembered for problems (see Eagleton, Thomas, and Palin, Sarah) than for boosting the ticket. Thus the watchword from the Romney camp is reportedly boring—"an incredibly boring white guy," a source told Politico.
Blandness is an odd quality to seek in a potential future presidential nominee—one quarter of the vice presidential nominees since 1900 have later been nominated in their own right, according to Silver. It's a pity that, in their wisdom, the founders couldn't foresee the problems with their "valuable mode of election" and dispense with it altogether.