An attorney for Lt. Gov. Justin E. Fairfax said a witness can corroborate the lieutenant governor’s claim that he did not rape a woman while they were students at Duke University nearly 20 years ago.

Fairfax lawyer Barry Pollack sent a letter July 9 to a North Carolina prosecutor saying the unnamed witness “has stated unequivocally” that the allegations made against Lt. Gov. Fairfax are false. Once a rising star among Virginia Democrats, Lt. Gov. Fairfax has fended off widespread calls for his resignation after two women came forward publicly in February accusing him of sexual assault years ago.

The lieutenant governor has said both encounters were consensual and asked for investigations by prosecutors in Boston and North Carolina, where the accusers said the assaults occurred. Both women have called for a bipartisan hearing at the Virginia General Assembly, which Lt. Gov. Fairfax and Democratic lawmakers have opposed.

Meredith Watson has said Lt. Gov. Fairfax carried out a “premeditated and aggressive assault” in a Duke University fraternity house in 2000. But Mr. Pollack said a witness can disprove Ms. Watson’s story.

“The eyewitness observed Ms. Watson initiate a sexual encounter with Mr. Fairfax in the eyewitness’ room,” Mr. Pollack said in his letter to Durham County District Attorney Satana Deberry. “The eyewitness remained with Ms. Watson after Ms. Watson’s sexual encounter with Mr. Fairfax had concluded and Mr. Fairfax had left.” Mr. Pollack declined to identify the witness, who he said “recalls the events clearly and has shared his recollections with a number of people.”

Mr. Pollack also said Lt. Gov. Fairfax was disappointed that Ms. Deberry’s office had not responded to his requests to meet with her office and “has not ruled out pursuing other legal remedies for the severe harm that this false allegation has caused.”

Ms. Deberry declined to comment.

Ms. Watson’s attorney, Nancy Erika Smith, issued a statement challenging Lt. Gov. Fairfax to have the witness testify at the legislature.

“If Justin Fairfax wants the truth to come out, this secret witness should testify under oath, in public, along with Mr. Fairfax, both his victims and their witnesses,” Ms. Smith said. “Fairfax continues to fight a public hearing tooth and nail. That says it all.”

Lt. Gov. Fairfax has characterized the assault allegations as part of a racially motivated smear campaign intended to derail his political career, although both of his accusers are African-American. The allegations became public shortly after a racist photo in Gov. Ralph S. Northam’s medical school yearbook surfaced and nearly drove the governor from office, which would have made Lt. Gov. Fairfax the governor.

Lt. Gov. Fairfax recently announced he was leaving his Northern Virginia law firm to focus on “serving the people of Virginia.” He also has said he may run for governor in 2021.