This post from Roy Beck of the anti-amnesty NumbersUSA, which is further analyzed by Mickey Kaus here, goes into detail about which Senators flipped from pro- to anti-amnesty. Short version: Republicans were a lot more scared of a Tea Party primary challenge in 2012 than Democrats were of general election challengers. Sayeth Kaus:
Score one for losing Delaware Tea Partier Christine O’Donnell, who knocked off establishment pick Rep. Mike Castle (who voted for DREAM) in the GOP primary. Even score one for Alaskan Joe Miller. He probably alienated Republican Lisa Murkowski by beating her in the primary, and ultimately she won reelection anyway as a write-in. But that’s just one lost Senate vote. By my count, Miller’s primary coup may have helped gain around ten votes by terrifying GOP incumbents who might otherwise have been tempted by the prospect of a feel-good, bipartisan, MSM-approved pro-DREAM stand.
Beck also noted at least two Democrats, Conrad of North Dakota and McCaskill of Missouri, who voted for an amnesty despite coming from deep red states and being up for reelection in 2012. Those two seats should be big, juicy GOP takeover targets two years hence…