NEW YORK — Speaking in a halting voice, a Chicago-area woman accused Republican presidential contender Herman Cain on Monday 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.