"Come clean," Sharon Bialek challenged Cain at a news conference in New York at which she described herself as "a face and a voice" to support other accusers who have so far remained anonymous.
Cain's campaign swiftly denied Bialek's account. "All allegations of harassment against Mr. Cain are completely false," it said in a written statement.
Even so, Bialek's nationally broadcast appearance on cable television marked a new and - for Cain - dangerous turn in a controversy that he has struggled for more than a week to shed. An upstart in the presidential race, Cain shot to the top of public opinion polls in recent weeks and emerged, however temporarily, as the main conservative challenger to Mitt Romney.
Accompanied by her prominent lawyer, Gloria Allred, Bialek accused Cain of making a sexual advance one night in mid-July 1997, when she had travelled to Washington to have dinner with him in hopes he could help her find work.
She said the two had finished dinner and were in a car for what she thought was a ride to an office building.
"Instead of going into the offices he suddenly reached over and he put his hand on my leg, under my skirt toward my genitals," she said.
"He also pushed my head toward his crotch," she added.
Bialek said she told her boyfriend, an unidentified pediatrician, as well as a longtime male friend about the episode.
None of Cain's other accusers has provided details as graphic as Bialek's account. But Joel Bennett, an attorney who represents one of them, said her details were "similar in nature" to what his client encountered.
Allred, a prominent sex discrimination attorney with Democratic ties, moved preemptively to blunt any attacks on Bialek's motives. She described her client as a registered Republican, a single mother and a woman with a long and successful work history.
She also said Bialek "could have attempted to sell her story but chose not to do so," and knew that by stepping forward, she would receive scrutiny.
Court records indicate Bialek had financial difficulties a decade ago when she filed for bankruptcy protection and reported $4,500 in unpaid rent and $13,000 in outstanding credit card bills.
Current property records show she owns a house on an acre of land in a Chicago suburb.
Some of Cain's allies immediately made a target of Allred, a Democratic campaign donor, rather than focusing any anger on Cain's accuser.
Georgia state Sen. Joshua McKoon, who has endorsed Cain, accused Allred of "carnival theatrics" fueled by a partisan agenda.
"Her involvement makes it clear that it's a political smear job orchestrated by those on the left because there is nothing more terrifying than Herman Cain as the Republican nominee," the Republican lawmaker said.
But Doug Heye, a political consultant who is unaligned in the GOP race, said Bialek's allegations "are different because they involve a name and specific details."
He said Allred's involvement "is going to make some people disbelieve the charges out of hand because of the side show she creates. But Herman Cain has to be clear and convincing in his response."
Even before Bialek stepped forward, presidential rival Jon Huntsman and Mississippi Gov. Haley Barbour had publicly urged Cain to address sexual harassment allegations in greater detail.
It wasn't clear he would.
After spending much of last week denying accusations, he told reporters who sought to question him Saturday night, "don't even go there."
Cain had an evening appearance scheduled on the Jimmy Kimmel show, his only public event of the day.
According to lawyer and client, Bialek was employed for parts of 1996 and 1997 at the Educational Foundation of the National Restaurant Association, an industry trade group that Cain headed at the time. She said she first met him at an organization convention, interacting with him several times over the course of a few days.
After she was fired from her job about a month later, she said her boyfriend told her, "Herman seems to think highly of you. Why don't you contact him?"
That led to a trip to Washington about a month later, where she recalled that Cain upgraded her hotel room to a suite, and made his unwanted sexual advance in the car.
She said she asked Cain what he was doing, and recalled he replied, "You said you want a job, right?"
Given her experience and those of other accusers, "I want you, Mr. Cain, to come clean," she said. "Just admit what you did. Admit you were inappropriate to people."
She added: "Mr. Cain, I implore you: Make this right so that you and the country can move forward and focus on the real issues at hand." The denial from Cain's campaign was as unequivocal as the allegation.
"Just as the country finally begins to refocus on our crippling $15 trillion national debt and the unacceptably high unemployment rate, now activist celebrity lawyer Gloria Allred is bringing forth more false accusations against the character of Republican front-runner Herman Cain," it said.
"Mr. Cain has never harassed anyone."
Before Bialek stepped to the microphone, the allegations involved two women who had worked at the National Restaurant Association, both of whom filed sexual harassment complaints.
A third woman told The Associated Press last week that she considered filing a workplace complaint against Cain over what she deemed sexually suggestive remarks and gestures that included a private invitation to his corporate apartment.
A former pollster for the restaurant association has said he witnessed yet another episode involving a fourth woman.
Allred has represented several high-profile clients, including Amber Frey, a witness against convicted California killer Scott Peterson. Allred also represented a woman whom news reports accused of having an affair with golfer Tiger Woods.
"I consider sexual harassment the No. 1 problem in the workplace," she told the AP in an interview last week. "It denies equal opportunity in the workforce. If (women) don't protest it, they'll have to continue to suffer."
Associated Press writers Philip Elliott in Washington and Shannon McCaffrey in Atlanta contributed to this story.