Cain's precipitous decline comes as Sharon Bialek, a former employee of the National Restaurant Association, came forward Monday to accuse Cain of having harassed and groped her after a dinner in 1997. Bialek is the fourth woman to accuse Cain of misconduct, allegations that the campaign have strongly denied.

But Cain has built a bit of a cushion in the polls: the candidate remains a co-leader, with Romney, atop the GOP field, each garnering 21 percent of supporters.

Still, Cain's loss seems to be Newt Gingrich's gain. After early struggles that threatened to derail his campaign, Gingrich has found a niche as the self-proclaimed intellectual of the party, unwilling to attack fellow candidates. While Gingrich's "positive intensity score" was as low as 1 percent in August, he has rebounded to 14 percent in recent days.

That score outpaces not just other second-tier Republican contenders like Michele Bachman and Rick Perry, who share a score of 3 percent, but that of presumptive frontrunner Romney.