But five days into the scandal, it isn't killing him. It's making him stronger.
Yesterday, the Rasmussen Reports telephone survey of 1,000 likely Republican-primary voters - conducted Wednesday night after three days of negative news coverage about the Georgia businessman - showed Cain as front-runner for the first time. It had Cain at 26 percent, Mitt Romney at 23 percent and up-and-coming Newt Gingrich at 14 percent.
How to explain this? Clearly, many GOP voters - especially in the tea-party wing where the former pizza CEO draws most of his support - believe that the major victim of harassment here is Herman Cain himself.
Harassment by the liberal media, that is.
Conservative radio talk-show host Dom Giordano of WPHT said the allegations are "rousing the base. My callers resent what they see as a double-standard that [Bill] Clinton and particularly [John] Edwards were not scrutinized in the same way as Cain. They also see a bit of a parallel to Clarence Thomas" - whose 1991 Supreme Court nomination was nearly derailed by sexual-harassment charges.
Indeed, the nonstop drip-drip-drip of negative stories about Cain in the mainstream media has allowed the unconventional candidate to successfully tap into the deep strains of cultural resentment and anti-elitism that have fueled conservative icons from Richard Nixon to Sarah Palin.
He will continue to do so unless, or until, the headlines about the harassment allegations (yesterday it was revealed that one of the women received $45,000 from the trade association) become more difficult for Cain to manage.
Already the scandal has caused Cain, seeking to become the first black presidential nominee of the Republican Party, and friendly conservatives to interject polarizing issues of race into the campaign. Cain told Charles Krauthammer on Fox News this week that racism is "a bigger driving factor of his critics on the left," while radio talker Rush Limbaugh said the scandal arose because "it really is about blacks and Hispanics getting too uppity."
Eric Deggans of the St. Petersburg Times, who wrote a column earlier this week blasting right-wingers like Ann Coulter for calling Cain's treatment a "lynching," said conservative support might collapse if damning new facts come out. "If the truth turns out to be different from what he first admitted, I think that will hurt him," he said.
But Cain's rise in the GOP White House sweepstakes has defied any and all conventional wisdom so far. His experience as a rags-to-riches businessman and a talk-radio host count for a lot more with the tea-party movement than actual governing experience, which would have tainted Cain with the kind of past compromises that have hurt Romney. His proud know-nothingism on foreign policy - such as his lack of awareness that China has possessed nuclear weapons for nearly a half-century - is a feature, not a bug.
"There tends to be a yin and yang - people tend to look for qualities in the new guy that they didn't see in the old guy," said G. Terry Madonna, the political scientist and pollster from Franklin & Marshall College.
He said the tea party loves to contrast the style of Cain - a blunt-talking self-made businessman - with that of President Obama, a verbose law professor by way of the Harvard Law Review.
It's been a winning formula that so far has been immune even to a sexual-harassment scandal.
At least until the next news cycle.