NEW YORK — Speaking in a halting voice, a Chicago-area woman accused Republican presidential contender Herman Cain today of making an unwanted sexual advance against her more than a decade ago, saying she wanted to provide “a face and a voice” to support other accusers who have so far remained anonymous.

“Come clean,” Sharon Bialek challenged Cain, demanding he confess to any inappropriate behavior with her or other women.

Cain’s campaign instantly issued a denial. “All allegations of harassment against Mr. Cain are false,” it said.

Bialek’s appearance at a news conference marked a new and — for Cain — dangerous turn in a controversy that he had been trying to lay to rest.

She described an evening in mid-July 1997 when she had dinner in Washington, D.C., with Cain, whom she had contacted in hopes he could help her find a job. The two 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.

She said she told Cain to stop, adding that he did.

Bialek said she did not file a workplace complaint against Cain at the time because she was not employed.

She said she informed both her boyfriend, an unidentified pediatrician, and a longtime male friend.

Bialek appeared at a news conference alongside Gloria Allred, an attorney known for sexual harassment cases.

As if to blunt any attacks on her client’s motives, the attorney described Bialek as a registered Republican, and a woman with a long and successful work history.

Allred said Bialek was employed for parts of 2006 and 2007 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.

In mid-2007, Allred said, Bialek was fired from her job, and her boyfriend suggested she contact Cain in hopes he could help her find employment.

That led some time later to a trip to Washington, where Cain upgraded her hotel room to a suite, Bialek said, and the unwanted sexual advance took place after they had gone to dinner.

“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.”