If I were advising Barack Obama, I'd suggest that he should be spending time -- a lot of time -- in constantly wooing Dick Lugar and Olympia Snowe. Both of them appear to be iffy, at best, for renomination (here's the latest on Lugar). And that's before what's likely to be a bitter government shutdown fight in March, which will probably make the position of GOP moderates even more tenuous.
Both would be locks in the general election as long as they could get nominated, but that's more likely -- much more likely, in Lugar's case, I think -- on the Democratic side. Of course, the problem here is Arlen Specter's fate last year, when he switched parties and then was defeated in the Democratic primary. But Obama should be doing what he can to clear the way.
Is it plausible that they might switch? I think so, even though both have deep ties to the GOP, because their version of the party is close to extinct. And because, when it comes down to it, most politicians really like winning elections a whole lot more than they like losing elections. That hardly means it's a sure thing, but it does mean that it's probably worth some presidential time and energy. Especially since the payoff would be very nice for the Dems.
(Switching was discouraged by the Pennsylvania Senate results, but switching in the other direction seems entirely implausible. Does anyone think that Ben Nelson could win a GOP primary for Senate? I wouldn't give him a 10% chance).