A woman who said she worked on a 2001 campaign for Scott M. Stringer, the New York City comptroller who is now running for mayor, has accused him of sexually assaulting her 20 years ago.The woman, Jean Kim, now a political lobbyist, said at a news conference on Wednesday that Mr. Stringer, without her consent, “repeatedly groped me, put his hands on my thighs and between my legs and demanded to know why I would not have sex with him.”She said that Mr. Stringer warned her not to tell anyone about his advances, some of which she said took place during taxi rides.Mr. Stringer strenuously denied the allegations, and said that he and Ms. Kim had a consensual relationship over the course of a few months.Roughly two hours after Ms. Kim’s news conference ended, Mr. Stringer convened his own. Standing with his wife outside their Lower Manhattan apartment building Wednesday afternoon, he repeatedly characterized Ms. Kim’s allegations as “false” and “inaccurate.”“Sexual harassment is unacceptable,” he told a gaggle of reporters. “I believe women have the right and should be encouraged to come forward. They must be heard. But this isn’t me. I didn’t do this. I am going to fight for the truth because these allegations are false.”After he spoke, his wife, Elyse Buxbaum, came to the microphone and attested to her husband’s character.Ms. Kim said Mr. Stringer, who was then a state assemblyman running for New York City public advocate, had offered to make her the first Asian Democratic Party district leader on the Upper West Side, with one proviso.“You would have to prove yourself to me,” she recalled Mr. Stringer saying.Ms. Kim said she did not come forward earlier because she was “fearful of his vindictive nature and that he would retaliate against me and destroy my career in politics.” Her lawyer said that Ms. Kim faced less of a risk now that she was transitioning away from political work.Ms. Kim’s account, which was reported by Gothamist, comes roughly eight weeks before the June 22 mayoral primary. In the limited early polling that is available, Mr. Stringer is often in third place, behind Andrew Yang, the 2020 presidential candidate, and just behind Eric Adams, the Brooklyn borough president.Mr. Stringer’s campaign had recently started to gain more steam, as he won the endorsements of the United Federation of Teachers and the Working Families Party, as well as one of two endorsements from the New York chapter of Sunrise Movement, a group of young climate activists. “The Momentum Continues to Build” was a headline in a recent email his campaign sent to the news media.People who had spoken with Mr. Stringer’s team in recent days described a sense of having turned the corner after months of struggling to break through in a crowded primary field. But among allies and others in touch with his campaign, there was concern that the accusation would damage his chances.Indeed, by late Wednesday, he had lost the backing of State Senator Jessica Ramos, one of the earliest supporters of his mayoral campaign.“I am officially rescinding my endorsement of Scott Stringer for mayor,” she said. “This kind of behavior is unacceptable in any workplace, and those who have perpetrated such acts must be held accountable for their actions, not given bigger platforms.”In Ms. Kim’s remarks on Wednesday, she said that she met Mr. Stringer, who was not married at the time, in 2001, through an introduction by Eric Schneiderman, who was then a state senator. Mr. Schneiderman would go on to become New York State attorney general, before resigning amid allegations that he abused women.Mr. Schneiderman, who admitted to the misconduct, did not return requests for comment.Ms. Kim said that she was an unpaid intern for Mr. Stringer’s 2001 campaign for public advocate; Mr. Stringer later said that she was a campaign volunteer. At some point that year, she joined a West Side Democratic club in which he was involved.Mr. Stringer “inappropriately and relentlessly pursued a sexual relationship with me,” she said, adding in a statement that he “kissed me using his tongue, put his hand down my pants and groped me inside my underpants.”She said she decided to come forward because she was sickened by Mr. Stringer’s run for mayor, and his portrayal of himself as an ally to women.“I am coming forward now because being forced to see him in my living room TV every day, pretending to be a champion for women’s rights, just sickens me when I know the truth,” Ms. Kim said.She called on Mr. Stringer to resign and withdraw from the mayor’s race.Mr. Stringer said his relationship with Ms. Kim was friendly until 2013, when she wanted a job on his campaign for comptroller and did not get one. Mr. Stringer also said that Ms. Kim had donated to his political campaigns.Understand the N.Y.C. Mayoral RaceWho’s Running for Mayor? There are more than a dozen people still in the race to become New York City’s next mayor, and the primary will be held on June 22. Here’s a rundown of the candidates.What is Ranked-Choice Voting? New York City began using ranked-choice voting for primary elections this year, and voters will be able to list up to five candidates in order of preference. Confused? We can help.“Based on my understanding, she did not apply for any job on his 2013 campaign,” said Ms. Kim’s lawyer, Patricia Pastor, who said that it was part of Ms. Kim’s job as a lobbyist to make small donations to candidates.Stephen Levin, a Brooklyn councilman, said he had known Ms. Kim a long time professionally, and described her as a “a very nice, very good person.”“For someone like Jean, her entire career is in New York City politics,” said Mr. Levin, who is backing one of Mr. Stringer’s opponents, Maya Wiley. “So I have no reason to believe that she’s not telling the truth. Just like in elected office, for a lobbyist, your credibility is the most important thing.”Mr. Stringer, who has spent decades in politics, has cast himself as an ardent progressive in recent years. In the mayoral election, some observers view him as the most viable of the three left-wing options, along with Dianne Morales, a former nonprofit executive, and Ms. Wiley, a former counsel to Mayor Bill de Blasio.Ms. Morales was the first mayoral candidate to issue a statement condemning Mr. Stringer. She and other candidates — Ms. Wiley; Mr. Adams; Mr. Yang; and the former Wall Street banker Raymond J. McGuire — all expressed solidarity with Ms. Kim.Kathryn Garcia, another mayoral candidate and the former sanitation commissioner, called on Mr. Stringer to drop out of the race. So, too, did Shaun Donovan, a mayoral candidate and former federal housing secretary.Ms. Garcia noted that Mr. Stringer backs a zero-tolerance policy for sexual harassment and that in March, he called for Gov. Andrew M. Cuomo, who is facing several sexual harassment allegations, to resign.Mr. Stringer said that he believed his situation bore no resemblance to Mr. Cuomo’s.“The allegations against Governor Cuomo are serious and multiple and they are in the workplace,” Mr. Stringer said. “I don’t think we are in the same situation.”Several supporters of Mr. Stringer’s mayoral campaign — State Senators Alessandra Biaggi and Julia Salazar and Assemblywoman Yuh-Line Niou — issued a joint statement demanding “accountability.”“As survivors of childhood sexual assault, we believe survivors,” they said. “Our commitment to a harassment-free government, workplace, and society is steadfast, and our zero tolerance standard regarding sexual assault applies to abusers like Andrew Cuomo, if not more so, to our friends.”State Senator Liz Krueger, a Manhattan Democrat, said that she had dealt with Ms. Kim in a professional capacity.“I was surprised and disturbed,” said Ms. Krueger, who has not yet endorsed a candidate in the mayor’s race. “There’s no reason for me not to think of Ms. Kim as a credible person.”“Maybe it’s time for us to stop voting for men,” she added.