I have a hard time seeing him killing his Senatorial career in the crib by being the vote that kept Harry Reid as the majority leader. Maybe if he's already ruled out running for re-election. I think he'd personally prefer to caucus with the Dems, but in this scenario I think he'll try to paint himself as the Murkowsky from Kansas.
To be fair, it'll probably be nearly impossible for him to get reelected even if he caucuses with the Republicans. A random Republican congresscritter will come barreling out of the R primary and knock him out by being more conservative, yet not insane like a Brownback/Buck/Angle type.
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