Harry Enten rubbishes the idea that VP Rubio could help Romney in any meaningful sense:
If Mitt Romney is losing by 3 points nationwide, he would still would be very competitive in Florida based on recent electoral history. So picking Rubio as his VP running-mate would probably help clinch a state he'd be expected to win in any event; with the effect of merely narrowing his defeat in the electoral college vote. The correct strategy for any candidate is to pick a candidate from a state that is either dead even or slightly leaning toward your opponent. That's why Rubio would not be a golden ticket to an electoral college victory. And beyond helping Romney in Florida, there's probably little else Rubio could do.
Previous thinking along similar lines here.