If he tries to block Obama's agenda in any substantive way (i.e., filibustering or at least supporting one) then yes.
Otherwise, no reason to cost ourselves a seat. He'd certainly be the most liberal Republican senator since at least Jacob Javits.
More liberal than Chaffee?
I'd say so...there are other issues besides the war (and I'm no big fan of Lieberman these days, believe me....).
If someone can cite ACU or ADA statistics from years when they both served together contradicting that, I'll concede the point.