WASHINGTON — A fifth woman accused Roy S. Moore, the Republican Senate candidate in Alabama, on Monday of making sexual or romantic advances toward her when she was a teenager, as senior Republicans in Washington called for him to drop out of the race and threatened to expel him from the Senate if he wins.
The new accuser, Beverly Young Nelson, told a news conference in New York that Mr. Moore attacked her when she was 16 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 in a statement she issued at the news conference. She said Mr. Moore warned her that “no one will believe you” if she told anyone about the encounter in his car.
Hours earlier, Senator Mitch McConnell, the Republican leader, said Mr. Moore “should step aside” and that he believes the women who have accused Mr. Moore of sexual misconduct when they were teenagers.
“I believe the women, yes,” Mr. McConnell said at a news conference in Louisville.
Mr. McConnell also said that encouraging a write-in candidate to run in the Dec. 12 special election is “an option we’re looking at.”
Senator Cory Gardner of Colorado, speaking in his role as chairman of the National Republican Senatorial Committee, said that if Mr. Moore wins the special election on Dec. 12, he should be expelled from the Senate, “because he does not meet the ethical and moral requirements of the United States Senate.”
Mr. Moore, a judge who was twice removed from the state’s high court, first for refusing to remove the Ten Commandments from the Supreme Court grounds, then for refusing to accept gay marriage, responded defiantly. He showed no sign of leaving the race ahead of Alabama’s Dec. 12 special election date.
In an afternoon statement, Mr. Moore’s campaign described Ms. Allred as “a sensationalist leading a witch hunt, and she is only around to create a spectacle.” The statement, issued before Ms. Allred’s news conference in New York, denied again “any sexual misconduct with anyone” by Mr. Moore.
Republicans here and in Alabama have been up in arms over the accusations, published last week in The Washington Post, that Mr. Moore pursued sexual or romantic relationships with teenagers when he was in his 30s. The reports have upended a race in a state that has not elected a Democratic senator in 25 years.
In a fund-raising appeal, Mr. Moore reached out to his supporters with the subject line: “Mitch McConnell’s plot to destroy me.”
“Apparently Mitch McConnell and the establishment G.O.P. would rather elect a radical pro-abortion Democrat than a conservative Christian,” he wrote.
And Mr. Moore’s wife, Kayla Moore, lashed out in a Facebook post on Monday, complaining about “a witch hunt” in Alabama and claiming that “we are gathering evidence of money being paid to people who would come forward.”
“Washington establishment and Democrat Party will stop at nothing to stop our campaign,” she wrote. “Prayers appreciated…..”
But with Mr. McConnell now firmly against his election, Mr. Moore and his candidacy promise to deepen the divide between Republican leaders in Congress and the populist wing of the party that is standing by the Alabamian. Another Republican senator, Susan Collins of Maine, also called for Mr. Moore to drop out of the race on Monday.
Anxious Republican officials spent much of the weekend trying to determine what, if anything, they could do to halt Mr. Moore without simply turning over the seat. If Doug Jones, the Democratic nominee, wins, it would narrow the Republican advantage in the Senate to a single seat.
But if Mr. Moore stays in and goes on to win, it could leave Senate Republicans with the difficult question of whether to stop him from being seated or seating him and immediately moving to expel him from the chamber.
One idea now being discussed under this scenario, brought up by two different White House officials who spoke on condition of anonymity, would be for Gov. Kay Ivey of Alabama to immediately appoint Attorney General Jeff Sessions to what had been his seat when it becomes vacant again. Mr. Sessions remains highly popular among Alabama Republicans, but his relationship with President Trump has waned since he recused himself from the investigation of the role that Russia played in last year’s campaign.
Republicans in Washington, though, have not ruled out fielding a write-in candidate and some of the party’s outside groups were expected to conduct surveys this week of who would be most formidable prospect, according to officials familiar with the plans.
Democrats, who have been restrained about their prospects in such a conservative state, said that if additional women like the one joining Ms. Allred tell their stories, it would undermine Mr. Moore’s case that he is being smeared in a single newspaper article.
“The more people that come out of the woodwork, the more women with similar stories, the more credible it becomes,” said Zac McCrary, an Alabama-based Democratic pollster. “It’s going to become easier to see through Roy Moore’s non-denial denials.”
Democrats have begun raising money in earnest for Mr. Jones, a former prosecutor who sent Ku Klux Klansmen to jail for the infamous Birmingham church bombing of 1963 that killed four young girls.
At the news conference in New York, Ms. Nelson grew 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. Her boyfriend was late that night to pick her up after work, she said, and Mr. Moore offered her a ride home. Ms. Nelson said that instead of driving her home, Mr. Moore drove to the back of the restaurant, parked his vehicle and forced himself on her.
Ms. Allred displayed a yearbook she and Ms. Nelson said is signed by Mr. Moore. The attorney said that Ms. Nelson contacted her to “enlist my help and support in coming forward publicly” following the allegations by other accusers. Ms. Allred said that her law firm spoke to Nelson’s mother and sister, who verified Nelson’s story.
The New York Times has not independently spoken to the family. Ms. Allred said that Ms. Nelson is willing to testify under oath.
Ms. Nelson said that she and her husband supported President Trump during the 2016 election, an attempt to neutralize arguments that she is making the claims for political reasons. Ms. Allred also pre-empted attacks that she is acting with partisan interests.