Businessman Herman Cain’s planned news conference to address allegations of inappropriate conduct toward a former employee of the National Restaurant Association could well be his last, best chance to wrench an increasingly negative narrative back onto a better course.

Republican presidential contender Herman Cain addresses an audience at AEI(American Enterprise Institute) for Public Policy Research in this October 31, 2011 file photo in Washington, DC AFP PHOTO/Karen BLEIER

That shift is almost certainly due to the fact that one of the women — named Sharon Bialek — went public with allegations on Monday that Cain had put his hand under her skirt and asked her to perform a sex act.

Bialek’s allegations have set off increased calls from the GOP establishment and Cain’s rivals in the Republican presidential race for him to come clean about his involvement (if any) in these incidents.

“These are serious allegations and they are going to have to be addressed seriously,” former Massachusetts governor Mitt Romney told ABC’s George Stephanopoulos in an interview earlier today.

Former House Speaker Newt Gingrich sounded a similar message. “He has to explain the charges and do so in a way that’s convincing,” Gingrich told ABC and Yahoo. “He owes her that and he owes the American people that.”

And, Alaska Sen. Lisa Murkowski (R) said today that if the allegations against are true, Cain should drop out of the race. “If, in fact, there is substance to the claims — and now there are apparently four individuals that have been involved or who have come forward — if that’s the case, I think that his opportunity as a nominee are gone,” the senator told CNN.

For Cain then, today’s news conference amounts to a pivot point (at best) and a tipping point (at worst).

Let’s take the tipping point scenario first.

As we have written in this space before, Cain’s strategy — if what he is doing can rightly be described as a strategy — has been scattershot. He has, at times, said he knows nothing of the specifics of the allegations and, at others, made clear he knows more than a little. In terms of his tone, Cain has swung between combative and comedic.

In an interview Monday night on Jimmy Kimmel’s late-night show, Cain tried to defuse the situation with humor — telling Kimmel of Bialek: “At least it wasn’t one of the many that have the first name ‘Anonymous’.”

Any attempt to defuse this situation with humor at today’s news conference would almost certainly be an unmitigated political disaster for Cain.

While he continues to retain support within some pockets of the conservative movement — most notably from talk radio host Rush Limbaugh — the last 24 hours (post-Bialek) have shown that there is some dissension in the conservative ranks toward Cain.

If he turns in a performance that looks or feels something short of credible — or seems to not, as promised, address the charges “head on” — it’s easy to see things going rapidly downhill for him.

(And don’t forget that there is a presidential debate set for Wednesday night in Michigan where this topic will almost certainly come up again.)

How could Cain turn this moment into a positive pivot point for his campaign? It’s not easy but it is possible.

Cain still retains a significant amount of support among conservatives and polling suggests that there is a large chunk of the Republican electorate who don’t take the allegations all that seriously yet. (Worth noting: All of that polling was conducted before Bialek went public with her allegations on Monday afternoon.)

What Cain needs to do then in today’s news conference is to a) show that he takes the allegations seriously and b) discredit Bialek.

His campaign is well on its way to doing “b”, releasing information earlier today that casts Bialek as an attention-monger who has a history of litigiousness and has struggled to hold down a job.

“In stark contrast to Mr. Cain’s four decades spent climbing the corporate ladder rising to the level of CEO at multiple successful business enterprises, Ms. Bialek has taken a far different path,” reads the release.

And, in an interview earlier this afternoon with ABC’s Jon Karl, Cain previewed his message at today’s news conference — issuing a sweeping denial of the accusations made by Bialek and insisting he has no recollection of her at all.

“I can categorically say I have never acted inappropriately with anyone,” Cain told Karl. “Period.” Regarding Bialek, he added: “I don’t even know who this lady is.”

In flatly denying any sort of knowledge of Bialek, Cain is taking a calculated risk. If no proof emerges that can disprove that assertion, Cain could well look back on this as the moment when he turned the story around.

If, on the other hand, evidence comes out that Cain did in fact know/remember Bialek, it could be curtains for him.