Jonathan Bernstein blames his rise on the Republicans who rehabilitated his image:

Republicans in Washington knew exactly what Newt was, but for over a decade they've chosen to pretend otherwise, either because he was useful to have on TV attacking the Democrats, or just because that's how Republicans do things. So after treating him as a distinguished statesman and brilliant conservative politician for over a decade, despite clearly not believing it (as was shown the second he spiked in the polls in December), they're just getting exactly what they deserve.

Larison is more succinct:

Republicans are faced with an unfortunate predicament entirely of their own making: they can rally behind the Republican Bill Clinton to try to stop Romney, or they can nominate the liar.

(Photo: Former Speaker of the House Newt Gingrich greets supporters at Tommy's Country Ham House January 21, 2012 in Greenville, South Carolina. Former Massachusetts Governor Mitt Romney also campaigned at Tommy's Country Ham House just before Gingrich. South Carolina holds its pivotal presidential primary today. By Win McNamee/Getty Images)