I think Santorum is the favorite at this point, although there's not much point in handicapping the race so early. He was the strongest challenger last time, and the GOP usually goes with that. And the conservatives will demand one of their own.
No he's not. He's like Pat Buchanan, a social conservative reactionary to be the anti-establishment candidate (even though Santorum is VERY establishment in his non-social views). He's in a distant fourth or fifth right now behind Christie, Rubio, Bush, and even Ryan.
Remember when Pat Buchanan was a US Senator for 12 years?
Remember when McCain or Bush Jr. or Bush Sr. or Reagan got beat for re-election by nearly 17 points?
Don't want to compare, but Bush Sr. actually lost two Senate elections in row and not by a hair.
