Joe Biden denies sexually assaulting staff assistant Tara Reade
"I'm saying unequivocally: it never, never happened," he said of the allegations during a TV interview on Friday.
He asked for a search of the Senate archives for any record of a complaint Ms Reade allegedly filed at the time.
Ms Reade made a criminal complaint to police last month.
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She said she was a victim of sexual assault without naming Mr Biden. The police complaint, she said, was filed "for safety reasons only" as the statute of limitations for her claim had expired.
Mr Biden is running against Republican incumbent President Donald Trump, who has been accused of sexual misconduct by some 25 women.
The Democrat put out his statement before speaking on Morning Joe, a show on US cable channel MSNBC.
What are Reade's accusations?
She was working as a staff assistant to Mr Biden from 1992-93, when he was a senator for the US state of Delaware.
Ms Reade, now 56, says that in 1993 he forced her up against a wall in the halls of Congress, and put his hands under her shirt and skirt, penetrating her with a finger.