His clients have included Abner Louima, a Haitian immigrant assaulted with a broomstick by officers in Brooklyn in 1997, and Nicole Paultre Bell, the fiancée of Sean Bell, a black man who was killed by the police outside his bachelor party in Queens in 2006.

On Oct. 1, Mr. Rubenstein was at the Four Seasons restaurant for the gala 60th birthday party of a longtime friend, the Rev. Al Sharpton.

After the party, two women accompanied him back to his apartment. One of them, a 43-year-old board member of Mr. Sharpton’s civil rights group, the National Action Network, spent the night, after her friend left. A lobby security camera filmed their arrival.

Mr. Montgomery said that the woman began to feel “foggy” and lost consciousness after her friend left. She woke to discover Mr. Rubenstein sexually assaulting her, pinning her arms down, she claimed. Later that morning, Mr. Rubenstein’s driver drove her home. About 36 hours later, she went to a hospital and reported what had happened to the authorities.

Benjamin Brafman, a lawyer for Mr. Rubenstein, said on Monday the woman had falsely accused Mr. Sanford. “What happened in this case was consensual sex between two consenting adults who were fully alert and fully awake throughout,” he said. He added, “Rape is undoubtedly a serious offense; to falsely accuse someone of rape, however, is equally offensive.”

Mr. Montgomery said the woman had filed a lawsuit against Mr. Rubenstein on Monday morning. Mr. Brafman said his client would countersue for defamation of character.

A critical question for prosecutors was whether the woman had been given a drug without her knowledge that would have incapacitated her, beyond the alcohol she willingly drank at the party. But law enforcement officials said toxicology tests on her blood showed nothing unusual except for traces of marijuana, and those were not in concentrations high enough to render her incapable of saying no to a sexual advance.

Mr. Montgomery, however, said on Monday the woman did not use marijuana. He said she recalled eating a cookie shortly before she lost consciousness and marijuana cookies were recovered from the apartment. “We knew there was something else going on besides alcohol,” he said. “Her toxicology results say she has marijuana in her system, and she does not use or smoke marijuana.”