There are two contradictory rules in politics about how to handle an opponent's embarrassing scandal.
Door Number One: Pile on. The guy's not "under fire" unless there are flames on him. In the words of James Carville, "if he's drowning, toss the sonuvabitch an anvil."
Door Number Two: Back away. The guy's on fire: You don't want to get singed!
The choice that Cain's opponents are making tells us a lot about how he can move past this. They're all picking Door Number Two. Rick Santorum, in Iowa, isn't talking about it. Ron Paul is explicitly saying he won't go after Cain's "character." The Perry and Romney campaigns merely deny that the story came from them.
Why not take a swing? Well, the story came from Politico, which (for today) conservative talkers and opinion-makers are labelling an organ of liberal bias. There are no names attached to the story, for good reasons, but that factor gives a rival candidate pause if he's thinking of launching an attack. Oh, yeah -- and then there's the fact that the NBC News/Wall Street Journal poll, which asked Republicans where their votes would go if Cain dropped out, doesn't particularly help anyone as much as it helps Romney, whom the Tea Party doesn't like (generally speaking), and Gingrich, whose niche campaign has remained sort of interesting, but remains a terrible anti-Romney for all kinds of reasons. (No money, marital baggage, inability to stick to a message for more than four weeks, horrible general election polls.) These factoids were confirmed by the DMR poll, in which Gingrich and Romney were the second-choice candidates of the most voters -- Cain, pre-scandal, was the poll leader and the second-choice favorite.