WASHINGTON — An Alabama woman accused Roy S. Moore on Monday of sexually assaulting her when she was 16, the fifth and most brutal charge leveled against the Republican Senate candidate. Senate Republicans are now openly discussing not seating him or expelling him if he wins the Dec. 12 special election.

The new accuser, Beverly Young Nelson, told a packed news conference in New York that Mr. Moore attacked her when she was a teenager and he was a prosecutor in Etowah County, Ala. Ms. Nelson was represented at the news conference by Gloria Allred, a lawyer who has championed victims of sexual harassment.

“I tried fighting him off, while yelling at him to stop, but instead of stopping, he began squeezing my neck, attempting to force my head onto his crotch,” Ms. Nelson said, growing emotional as she described the assault, which she said happened one night after her shift ended at a local restaurant, where she was a waitress.

She said that Mr. Moore warned her that “no one will believe you” if she told anyone about the encounter in his car.