Since 1900, only one Republican became president by defeating an incumbent. That Republican was Reagan. He had four traits that might have helped him:
1. He was conservative.
2. He was a governor.
3. He sought the nomination, in a previous election.
4. He was from the West.
The Republicans, who might run, who have at least three of those four traits, are Sarah Palin, Dirk Kempthorne, Mike Huckabee, and Mitt Romney. I hope that all of them will run, but I don’t know whom I would support.
I’m not sure old models apply. Until Obama, senators had no success seeking the presidency. There just aren’t that many western governors who stand out, while the crises are really in the midwest and east. There, you already (though it’s REALLY early) have Christie, Scott, and Walker, with Kasich’s turn at bat coming. But I don’t think there’s enough lead time for these guys.