When we first caught a glimpse of Newt Gingrich’s hand poking out of the smoldering crater of his candidacy — remember way back to last spring? He had a bajillion-dollar account at Tiffany’s, took a Greek isles vacation, his whole staff quit? — I was in a state of disbelief. (My exact words:  “Parties don’t nominate people like that. You nominate a telegenic front man, not an erratic, overbearing, morally repulsive tub of goo like Gingrich.”) Then I started to give him a chance at winning, and he cratered, first in Iowa, and then everywhere.

But now Gingrich is leading in South Carolina. And, according to Gallup pollster Frank Newport, he is once again gaining on Mitt Romney nationally.

The most common explanation here is that Gingrich relaunched his foundering campaign at the debate Monday night when he lit into Juan Williams by turning his own racial dog whistle (Obama as the “food stamp president”) into a rant against omnipresent political correctness that apparently pervades even such bastions of conservatism as Fox News. That seems as good an explanation as any.

What I don’t quite understand is how this could stop, and even reverse, the momentum Romney has built up since New Hampshire. He had been riding a wave of positive news – victories, endorsements – and burying his opponents under a pile of cash. (He and his allies have outspent Gingrich 2-to-1 in South Carolina.) I have spent more than a year harping on Romney’s massive vulnerabilities against any plausible challenger, but he seemed to have run out of plausible challengers. Is a re-re-re-resurrected Gingrich, shot full of holes but still lurching forward, really plausible enough? It’s hard to see it, but the latest turn of events was hard to predict, too.