RICHMOND, Va. — Lt. Gov. Justin Fairfax of Virginia emphatically denied on Monday a woman’s claim that he sexually assaulted her in 2004, suggesting at one point that Gov. Ralph Northam’s supporters were trying to block his ascent to the governorship at a moment when Mr. Northam is besieged by demands that he resign over charges of racism.

“Does anybody think it’s any coincidence that on the eve of potentially my being elevated that that’s when this smear comes out?” Mr. Fairfax told reporters surrounding him in the rotunda of the state Capitol about whether he believes Mr. Northam, a fellow Democrat, was behind the accusation’s coming to light.

He softened his suggestion as he left the Capitol Monday night, telling reporters he had “no indication” that Mr. Northam was responsible.

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But in the same conversation, Mr. Fairfax hinted that Levar Stoney, the mayor of Richmond and a potential rival to Mr. Fairfax for the 2021 Democratic nomination for governor, may have played a role — praising the acumen of a reporter who inquired whether Mr. Stoney might have been responsible.

Asked if he had any involvement in leaking the claims of assault, which first surfaced Sunday night on a right-wing website, Mr. Stoney said, “The insinuation is 100 percent not true, and frankly it’s offensive.”

Just hours after The Washington Post published a story on Monday outlining the woman’s allegation — that Mr. Fairfax assaulted her soon after they had met in Boston at the Democratic National Convention — the lieutenant governor said that he and the woman had had what he called “a 100 percent consensual” sexual encounter.

“We hit it off, she was very interested in me, and so eventually, at one point, we ended up going to my hotel room,” Mr. Fairfax said, recounting in a measured voice what he said had happened in 2004.

Mr. Fairfax asserted that the woman had subsequently called him and said she wanted him to meet her mother. He said he had no documentary evidence of any further conversations after their sexual encounter in 2004, when he was working as the personal aide to John Edwards, then a senator from North Carolina and the Democratic vice-presidential candidate that year.

Asked if he had seen her since the time in his hotel room, he said, “I don’t believe so.”

The New York Times has reached out to intermediaries for the woman who made the allegation, but they did not immediately comment on Monday.

An adviser to Mr. Northam, asked Monday if the embattled governor had been behind the revelation of the assault allegations, denied any responsibility and said the Northam camp did not have the capacity to plot such a move at a moment when Mr. Northam is struggling to retain his job.

The accusations against Mr. Fairfax threw Virginia’s government into a deeper state of chaos three days after an image surfaced from Mr. Northam’s medical school yearbook page showing a man in blackface and another in a Ku Klux Klan robe. The governor, after initially acknowledging he was one of the men in the 1984 photo, is now denying that he was and refusing insistent calls from state and national Democrats that he resign.

Even though Mr. Northam admitted Saturday at his own news conference to once darkening his face with shoe polish for a Michael Jackson costume at a dance party, his advisers said Monday that he would not step down, and his chief of staff told other aides at a staff meeting that they owed it to Mr. Northam to remain with him for now, according to a Democrat briefed on the conversation.

For his part, Mr. Fairfax noted that he was in “a unique position” and wanted to remain “circumspect” about whether he thought the governor should quit.

But he was less restrained in claiming that there was a “manipulation” effort to derail his career in elective politics, which began with his election as lieutenant governor in 2017.

“It’s such a shame that this has been weaponized and used as a smear, because this is a very real issue,” he said of sexual assault.

Mr. Fairfax ran separately from Mr. Northam during the Democratic primaries for lieutenant governor and governor; unlike candidates in some other states, Mr. Northam did not choose Mr. Fairfax as his running mate.

After Mr. Fairfax spoke to the news media for just over 10 minutes, the clerk of the State Senate, Susan Clarke Schaar, strode through a pack of reporters and photographers and told the lieutenant governor he needed to return to his post presiding over the chamber.

Monday’s spectacle beside the Houdon-designed George Washington statue in the Capitol was extraordinary: Mr. Fairfax, a 39-year-old African-American, rebutting claims of sexual misconduct and suggesting the accusations were part of an effort by allies of Mr. Northam’s to salvage a governorship that is listing due to the racism that still scars Virginia.

The fast-moving events began at 2:55 a.m. Monday, when aides to Mr. Fairfax — a Democrat widely seen as a rising star in the party — put out a statement saying the allegation was “false” and that Mr. Fairfax had “never assaulted anyone — ever — in any way, shape or form.” The aides said that Mr. Fairfax was considering “appropriate legal action against those attempting to spread this defamatory and false allegation.”

The Post story Monday said the woman contacted the newspaper after Mr. Fairfax won election in November 2017 and alleged that he had sexually assaulted her in 2004.

The Post was unable to corroborate her allegations, which Mr. Fairfax had denied, according to Monday’s story. The Post, however, disputed the Fairfax statement’s assertion that the newspaper had found inconsistencies and red flags in the woman’s allegation.

Fairfax aides issued a second statement on Monday afternoon, this time accusing The Post of having “smeared” Mr. Fairfax by publishing its Monday story detailing the allegations without corroboration. The Post’s executive editor, Martin Baron, replied that Mr. Fairfax had gone first in “issuing a statement regarding allegations against him, making specific representations about Post reporting,” and that the paper had an obligation to clarify the nature of allegations and its reporting.

As Mr. Northam held meetings Monday, first with his cabinet and then with his staff in the state Capitol complex, stunned legislators arrived to word of the middle-of-the-night statement by the Fairfax aides after the publication of the story by a site calledBig League Politics.

After Mr. Fairfax, the second in line to become governor is the state attorney general, Mark Herring, a Democrat who had already indicated plans to run for governor in 2021.