Mitt Romney and Florida Sen. Marco Rubio will campaign together in Pennsylvania Monday, presumably fueling speculation that Rubio is being vetted to be Romney’s running mate. This isn’t the first time Romney has campaigned with a potential candidate for vice president. He’s appeared on stage at events with Ohio Sen. Rob Portman, New Hampshire Sen. Kelly Ayotte, former Minnesota governor Tim Pawlenty, Wisconsin Rep. Paul Ryan, and others. But today will be his first such appearance since Rick Santorum dropped out of the race, leaving him virtually unopposed in Pennsylvania.