Sen. Marco Rubio (R-Fla.) is a bookie’s favorite veep pick. (J. Scott Applewhite/AP)

One way to look at the question is to check in with the bookmakers. Paddypower , which gives odds on all manner of political bets, today opened betting on possible Number Twos on the Romney ticket. As of this posting, Sen. Marco Rubio (R-Fla.) is the top choice, with betters predicting his selection at odds of 11/4.

Rubio’s followed by New Jersey Gov. Chris Christie (odds of 5/1) and Virginia Gov. Bob McDonnell at 6/1. After that, it’s New Mexico Gov. Susana Martinez and Rep. Paul Ryan at 9/1; and Sen. Rob Portman and Sen. Rand Paul at 10/1.

Sorry, Herman Cain, your odds are a longshot 100/1.