Wouldn’t you know it? And just when things seemed to be going so well.

Here was Herman Cain, rolling along with a gigantic head of steam, his long-shot campaign for the Republican presidential nomination gaining momentum with every new opinion poll – and pow! He gets a bucket of ice-cold water right in the face.

Sexual harassment? Gee, he seemed like such a nice guy.

Cain has denied charges that he engaged in “inappropriate behavior,” the phrase used by Politico in reporting the allegations. Exactly what inappropriate behavior the alleged harassment consisted of was not reported.

The recently emerged co-front-runner in the GOP presidential race didn’t do himself much good when the story first broke, initially saying he didn’t know anything about the charges and then progressively changing his version of events as the story seeped into the round-the-clock circus we call the news cycle.

No one should expect Cain to admit doing something he didn’t do, but if he’s innocent he has a strange way of showing it. First, he didn’t remember. Then he remembered something. Then he remembered something else.

That kind of evolving story sounds like a lie, even if it isn’t.

Whatever happened when he was head honcho at the National Restaurant Association back in the 1990s, it surely must have been enough to sear itself in his memory. Two women accused him of sexual harassment and the organization he ran paid them off to go away and shut up about it. Who wouldn’t remember that?

Being charged with sexually inappropriate behavior would surely be upsetting, even humiliating, for anyone – but even more so for a powerful business executive who harbored political ambitions. The only way such an experience might slip someone’s mind, we’d think, would be if such charges were so commonplace that they became forgettable.

There is no evidence that Cain was a serial harasser, there’s only the word of Politico and its anonymous sources, in fact, that he did anything at all that was questionable or inappropriate. On Wednesday, however, the Associated Press reported that yet another woman who worked for the National Restaurant Association was accusing Cain of sexual harassment.

Once the floodgates open …

It’s not surprising that a man who entered the presidential race with little fanfare and limited national recognition would suddenly become a target for damaging allegations after his sudden surge to prominence. What is surprising is that Cain seemed so thoroughly unprepared to deal with the allegations.

The intense scrutiny that is part and parcel of a high-profile presidential campaign has caught many a candidate flat-footed – Texas Gov. Rick Perry still looks like a guy who doesn’t know what hit him – but an aspiring commander in chief with a potential skeleton in his closet should at least have an explanation ready to go in a file drawer or email folder labeled “just in case.”

Unfortunately for Cain, he apparently didn’t plan for such a contingency and now he’s scrambling to wiggle out of the trap that ensnared him.

The fact is, with or without these allegations, Herman Cain was not going to be the Republican nominee for president. Recent polls notwithstanding, former Massachusetts Gov. Mitt Romney remains the real front-runner and Perry figures to be his most persistent challenger, if for no other reason than he has the fundraising wherewithal to continue the fight through the primaries.

Cain’s out-of-nowhere charge to the front of the pack would have fizzled eventually, no matter what. He just doesn’t have the credentials to be a serious candidate for president.

But if he wants to keep his campaign going, at least for a while, Cain needs to get in front of this story. If he did what he’s accused of doing, he needs to come clean, make a full confession and a public apology – surely he remembers Bill Clinton admitting he caused pain in his marriage in that famous “60 Minutes” interview in 1992. Even if Cain didn’t harass anyone, he needs to submit to a no-holds-barred press conference or interview where he takes on every possible question and answers every allegation in painful detail.

Facing the music, for better or worse, might allow Cain to derail the story and move on. Or not. But he has no chance of getting past it if he doesn’t confront it head-on, once and for all.

On the other hand, he could drop out of the presidential race, launch a speaking tour, write a book and spend his spare time appearing on TV talk shows. There are worse ways to make a living.