David Letterman started off his 'Late Show' (weeknights, 11:35PM ET on CBS) interview with Herman Cain by making fun of himself. "I'm no stranger to sexual scandal," he said, which drew a big chuckle from Cain. But then Letterman got serious about the sexual harassment allegations that have been leveled against Cain by several women, asking him if he would drop out of the race if they were proven true.

"One of two things will happen ... you'll be exonerated just like Justin Bieber ... or they will prevail and there will be somebody else coming forward" with documentation and more evidence. "Now will you resign if that's the case?" he asked.

Cain continued to deny that he had done anything wrong. "Let's be very clear about the allegations that have already been made. The reason that I went front and center in front of the media is that I knew there wasn't anything to it. There's no further documentation has come up. So we have moved on."

Then Letterman got even more incredulous. "So these statement are all false. All of these women are lying. They're all lying. They are all lying," he repeated.