Joe Biden has denied a former staffer’s allegation of sexual assault, saying the accusation is “not true”.
“This never happened,” Mr Biden said in a statement on Friday.
The statement is Mr Biden’s first public comment on an accusation of sexual assault by his former Senate staffer, Tara Reade.
“I’m saying unequivocally, it never, never happened,” Mr Biden said in an interview on MSNBC’s Morning Joe.
Mr Biden said he will ask the National Archives to determine whether there is any record of such a complaint being filed.
“The former staffer has said she filed a complaint back in 1993,” Mr Biden said.
“But she does not have a record of this alleged complaint.
“The papers from my Senate years that I donated to the University of Delaware do not contain personnel files.”
Mr Biden, who served as vice president to Barack Obama, said: “There is only one place a complaint of this kind could be, the National Archives.
“The National Archives is where the records are kept.”
The presumptive Democratic nominee for the presidency is expected to duel with Donald Trump for the keys to the White House in November.
Republicans worried about Mr Trump’s increasingly precarious political standing are seizing on the allegation to portray Democrats as hypocrites who only defend women who allege wrongdoing against conservatives.
They are digging in despite the fact that it could renew attention on the multiple sexual assault allegations lodged against Mr Trump.
Democrats, meanwhile, are in an awkward position of vigorously validating women who come forward with their stories while defending the man who will be their standard-bearer in what many in the party consider the most important election of their lifetimes.
“The campaign has issued statements, but he hasn’t issued any statements in his own voice,” said former Democratic National Committee Chairwoman Donna Brazile.
“It’s not helping, it’s just damaging — not only to the person who has come forward, but it’s also damaging the candidate.”
The November contest between Mr Biden and Mr Trump will be the first presidential race of the #MeToo era, which has led numerous women to come forward with allegations of sexual assault.
Mr Trump has been accused of assault and unwanted touching by numerous women, allegations he denies.
Women are a core constituency for Democrats, and Mr Biden has a mixed history.
While he wrote the Violence Against Women Act as a senator, he also came under heavy criticism for his handling of Anita Hill’s Senate evidence in the 1990s.
Just before he launched his 2020 campaign, several women accused him of unwanted touching, behaviour for which he apologised.
Mr Biden has pledged to pick a woman as a running mate, and the allegation has left those thought to be in contention in a tough spot.
Stacey Abrams, the former Georgia Democratic governor candidate, said: “I believe Joe Biden”, citing a New York Times investigation that she said exonerated him.
“Women deserve to be heard,” she said, “but I also believe that those allegations have to be investigated by credible sources.”
Republicans say the Democrats are not being consistent, pointing to aggressive questioning and coverage of Supreme Court nominee Brett Kavanaugh when he faced an allegation of sexual assault.
Speaking about the allegation for the first time on Thursday, Mr Trump said Mr Biden “should respond” before proceeding to criticise the treatment of Mr Kavanaugh as “an absolute disgrace to our country”.
Steve Guest, a spokesman for the Republican National Committee, said “the left, and their media allies, has one standard for Republicans and another standard for Democrats like Joe Biden”.
“The double standard,” he said, “is appalling.”