Herman Cain has been leading in the Republican presidential polls for a while now, yet few who cover politics for a living believe that he can stay there. Karen Tumulty, in a Sunday piece in The Washington Post about a recent surge by Newt Gingrich, wrote these prescient words about Cain:

One by one, hot new al­ternatives to [former Massachusetts governor Mitt] Romney have arisen and stumbled: first Minnesota Rep. Michele Bachmann, then Texas Gov. Rick Perry. For­mer Godfa­ther’s Pizza chief exec­utive Herman Cain has shot up in the polls, though his vulnera­bilities become more ap­par­ent with ev­ery news cycle.

Those words, of course, hit the Internet long before Sunday night, when Politico dropped its bombshell on allegations that Cain, in a previous job as head of the National Restaurant Association, had kicked up sexual harassment complaints from at least two female employees. More vulnerabilities, indeed.

The Politico story leaves Cain weakened on two levels. One is the substance of the story: that he allegedly handled professional relationships with two women in such a way as to prompt complaints of sexual harassment. Though Politico doesn’t have granular details on the alleged behavior, it reported that one woman under Cain on the NRA org chart told others that she had “suffered what the source described as ‘an unwanted sexual advance’ from Cain at a hotel where an event involving the group was taking place.”

That right there is a big deal.

A deal of comparable magnitude relates to how Cain has handled this whole mess. In the story, Politico notes that Cain, when apprised of these claims, reported that he was “vaguely familiar” with them. Here’s the relevant portion of the Politico story:

Cain spokesman J.D. Gordon told POLITICO the candidate indicated to campaign officials that he was “vaguely familiar” with the charges and that the restaurant association’s general counsel had resolved the matter.

In a journalism shop that’s what we call “confirmation.” As in, copy-edit the thing, fact-check it, and hit “publish.”

Yet Politico treated this powder keg of a story with utter sensitivity to its explosiveness. It didn’t just accept a statement from a flack — second-hand stuff. Rather, it decided that the story was big enough that it needed to put the questions directly to the man himself. And in executing that final step, Politico showed why doing so is good not only for the integrity of the story, but also for the drama of the story.

Politico reporters caught the ever-more-popular candidate Sunday morning outside the Washington bureau of CBS News, where Cain had come for an appearance on “Face the Nation.” Here’s part of the encounter that took place, as described in the story:

He was then asked, “Have you ever been accused, sir, in your life of harassment by a woman?”

He breathed audibly, glared at the reporter and stayed silent for several seconds. After the question was repeated three times, he responded by asking the reporter, “Have you ever been accused of sexual harassment?”

We knew that Herman Cain was proud; we knew he was a good orator; we knew he was eccentric. But did we ever think he was a bully?

Now the Cain campaign has taken the next step, confirming to the Associated Press that it is now denying the story. Gordon issued this statement:

Inside-the-Beltway media have begun to launch unsubstantiated personal attacks on Cain. Dredging up thinly sourced allegations stemming from Mr. Cain’s tenure as the Chief Executive Officer at the National Restaurant Association in the 1990s, political trade press are now casting aspersions on his character and spreading rumors that never stood up to the facts.

This is really great stuff, the sort of spin that makes a media blogger happy to be on shift on a Sunday night. Take a look for a minute at the whole notion that the Politico story was “thinly sourced.” At first blush, it appears to be a fine talking point for the Cain campaign. After all, there are indeed plenty of anonymous sources in the story, alleged harassment victims bound by nondisclosure agreements and other folks who didn’t want their names used.

Yet who was the most authoritative, on-the-record source? Republican presidential candidate Herman Cain. He was the guy who said he was ”vaguely familiar” with the allegations. When a guy running for president says he has a vague recollection of something bad that happened in the past, he has a stupendous recollection of the event. That’s just the rounding error for politicians.

So if the Politico story is “thinly sourced,” it’s so only because it’s main, on-the-record source has very little credibility.