Massachusetts:
Barack Obama 62%
John McCain 36%
...And yet Scott Brown won just a year later. While I grant you that Brown is more liberal than O’Donnell obviously (especially on many social issues), he campaigned as a candidate decidedly more conservative than Castle.
Further, Brown ran as the 41st vote against Obamacare; now that Obamacare is even more unpopular than it was then, why is Castle saying that he won’t vote in favor of repealing it if he’s Senator?
Barack Obama 62%
John McCain 36%
Scott Brown is more conservative than Castle, and more liberal than O'Donnell. He is also an especially attractive candidate. If it was a Scott Brown running, then sure, give Castle the boot, because Brown would win in the General. But O'Donnell is no Scott Brown, and may well lose in November.
The Republicans have got to win with what we've got. In the Blue states, RINO-esque candidates will emerge. Castle in DE, Kirk in IL, Fiorina in CA. We deplore their RINO'ism, but for these States it is likely the best we are going to get. Running tea party heartthrobs makes sense in Red states like AZ, Alaska, Utah, but is suicidal in the Blue States.