Now it appears Cain's campaign is in ruins. A woman came forward claiming a 13-year affair with Cain. She was able to produce some evidence of a personal connection to Cain, which Cain did not deny. Cain did deny that the affair took place. Unfortunately for Cain, this was one denial too many. Only the most fanatic Cain supporters will continue to believe that he is a serviceable general election candidate.
Early reporting suggests that Cain is considering whether or not to drop out of the race. Even if he does not quit, the fact that he considered it will also be counted against him. Herman Cain's odds of winning the nomination are now 0%, according to Elephant Watcher's most recent recalculation. Though Cain was at one point leading all of the national polls (and Iowa and South Carolina polls), Elephant Watcher never gave Cain more than a 5% chance of winning the nomination. Cain was weighed down too heavily by his inherent weaknesses as a candidate. Now those weaknesses have doomed his campaign.
Ironically, the person likely to benefit most from Cain's demise is Gingrich, who admitted to having affairs during both his first and second marriages: The Cain supporters who already left Cain largely went to Gingrich, and Mitt Romney obviously prefers to split his opponents' vote as many ways as possible. Cain's further drop is critical because it will push his numbers below the threshold at which his support will disappear entirely. Voters tend to abandon a candidate if his vote is not above, say, 10%, so that they may coalesce around their second choice.
Gingrich continues to be weighted down by issues of his own. Though he has inherited Cain's lead in the polls, and while he is better suited to win New Hampshire votes than Cain was, Gingrich is completely unvetted. No candidate has yet attacked Gingrich. For Gingrich's odds to rise with his poll numbers, he must first demonstrate that he is capable of surviving the same level of scrutiny that destroyed Cain.