Victor Jay Zuckerman's account of an evening he, Sharon Bialek and Cain spent together in 1997 directly contradicts the candidate's assertions that he had never met his accuser or heard her name.
"At that party, Mr. Cain engaged both of us in conversation," Zuckerman said at a news conference, describing an after party Cain had invited them to in a hotel suite after a National Restaurant Association event in Chicago.
Cain was chief executive of the Washington trade group at the time.
Attorney Gloria Allred, who represents Bialek, appeared at Zuckerman's side and called on Cain to acknowledge that he had met his accuser, one of at least four who have alleged that Cain sexually harassed or made unwanted advances toward them.
"Mr. Cain's strategy of blanket denials simply won't work," Allred said. "He needs to come clean with the American people. Now is the time."
Zuckerman said Bialek told him that Cain inappropriately touched her later that year when she met him in Washington to seek employment help after being fired from the association.
Cain said last week that he had never seen Bialek until she went public.
"I don't even know who this woman is," he said. "I tried to remember if I recognized her and I didn't."
Cain has been dogged since late last month by sexual harassment accusations by former association employees. The controversy was entering its third week Monday as the Louisiana pediatrician stepped forward to corroborate some of Bialek's allegations.
Zuckerman did not, however, witness the alleged sexual advances. He could only confirm that they and Cain had met.
"I can confirm that when she returned, she was upset," Zuckerman said.
There also was fresh evidence that accusations are causing Cain to lose public support. Some surveys show him dropping from the top of the polls, where he had been in the weeks before the first of the decade-old accusations surfaced.
Cain has defiantly denied any wrongdoing but he also has been unable to put the questions behind him with less than two months until Iowa's leadoff caucuses.
Cain has vowed to stay in the race for the GOP nomination and has deployed his wife of 43 years to defend him. Gloria Cain was appearing in a television interview set to air Monday night after being absent from the campaign trail for much of the year.
"I'm thinking he would have to have a split personality to do the things that were said," she said in excerpts of the Fox News Channel interview that were released Sunday.
She said she cannot believe the claims.
"To hear such graphic allegations and know that that would have been something that was totally disrespectful of her as a woman and I know that's not the person he is," Gloria Cain said. "He totally respects women."
That conflicts with the image Bialek has described.
The Chicago resident became the first woman to publicly accuse Cain, a Georgia businessman, of inappropriate sexual conduct in the late 1990s when he was chief executive of the National Restaurant Association.
Bialek had lost her job in the association's Chicago office and had gone to Washington to meet Cain for help finding employment. The two had dinner and afterward, as they sat in a parked car, Bialek said Cain groped her.
At least three other women have claimed that Cain sexually harassed them. Bialek is the only one of the four to go public with their accusations.