Rubio - Not enough experience. Also reminds Conservatives of why they don't like Romney to start with.
Portman- Probably the strongest possibility. Doesn't upstage him, could actually run the country if Romney makes an unexpected trip to the Celestial Heaven, helps him in a key state.
Chris Christie- Really, the guy is too unpleasent. You don't want a ticket with two people no one really likes.
Pawlenty- Probably a good pick, but he flopped so badly in 2007 he's toxic.
Jindal- Anyone remember when he gave the counter to the SOTU address, and flopped?
McDonnel- two words- Transvaginal Ultrasound.
Paul Ryan - What, Darth Vader wasn't available?
Mitch Daniels- Will remind conservatives why they don't like Romney.
Toomey- Too far out on the fringe.
Ayotte, Rodgers, Martinez - Again, Palin totally screwed that up for any female running mate who doesn't have a boatload of expereince/accomplishments in her own right.
Condi Rice- A good pick, but she's not interested in politics. Also, probably not interested in getting into a black against black thing with Obama.
Thune- Good for the reasons Portman is, but the downside- he doesn't bring a state with him that Romney needs.
Jeb Bush- He's just waiting for Romney to do a Bob Dole so he can do his own thing in 2016.
Martinez- Not enough experience.