Governor Crist enjoys broad bipartisan support in Florida. He supported President Obama's stimulous bill. The only problem is he likes Floridians and acts in their interest. The Club for Growth who funds most Republicans hates him. They will have to pour money into the state running someone against Crist in the primary and if they win the primary they'll be sure to lose the seat to a Dem. More likely Crist will win the primary in which case no Dem could take him out, and there would be a very moderate Republican often voting with the Dems.
Heads win / tails lose.