US Supreme Court nominee Brett Kavanaugh has dismissed a third woman's accusation against him of misconduct as "ridiculous".
The new claim, which is being investigated by the Senate, alleges Judge Kavanaugh committed serious sexual assault in high school.
In a statement, the judge said he did not know the accuser, Julie Swetnick, and her allegations "never happened".
He is set to testify on Thursday over other sexual misconduct claims.
In a sworn affidavit, Ms Swetnick alleged Judge Kavanaugh was involved in the drugging and sexual assault of girls at house parties in the 1980s.
She says she was the victim of a gang rape in 1982 at a party attended by the judge.
Ms Swetnick is represented by lawyer Michael Avenatti, who has declared his ambition to run for president as a Democrat in 2020.
His most famous client is porn actress Stormy Daniels, who says she had an affair with President Donald Trump.
Lawyers for the Senate Judiciary committee say they are reviewing Ms Swetnick's testimony.
Mr Avenatti has said his client is willing to testify before the panel.
It comes just a day before the first accuser to come forward, Christine Blasey Ford, is due to outline allegations of sexual assault by Brett Kavanaugh at a high school party more than 30 years ago.
Another woman, Deborah Ramirez, a former Yale University classmate, has said that the nominee once exposed himself to her at a dormitory party in the 1980s.
But the third allegation is more serious than the other two.
"This is ridiculous and from the Twilight Zone," Judge Kavanaugh said in a statement shared by the White House on Wednesday.
"I don't know who this is and this never happened."
The president responded to the new claims, tweeting that Mr Avenatti was a "third rate lawyer who is good at making false accusations".
Avenatti is a third rate lawyer who is good at making false accusations, like he did on me and like he is now doing on Judge Brett Kavanaugh. He is just looking for attention and doesn’t want people to look at his past record and relationships - a total low-life!— Donald J. Trump (@realDonaldTrump) September 26, 2018
In a meeting with the Japanese Prime Minister later on Monday, he told reporters: "I think it ridiculous. It's a con game that they're playing."
"He's a high quality person. They're bringing people out of the woods. They could do it to anybody," he continued.
"It's a horrible con game. I think the people are finding it out. And hopefully over the next couple of days it will be settled up and solved and we will have a Supreme Court justice who will go down as one of our greatest ever."
Who is the new accuser?
Julie Swetnick is a Washington DC resident who has spent her career working for the US federal government, including the US Department of Treasury, the US Mint and the Internal Revenue Service, according to her sworn affidavit.
In the legal document released on Wednesday, Ms Swetnick notes she holds several active clearances related to her work for the US government.
She says she attended high school in Gaithersburg, Maryland, and has received university degrees from Montgomery College and the University of Maryland.
Her lawyer, Mr Avenatti, called her "courageous, brave and honest", and appealed for privacy for her and her family.
What's the reaction?
All 10 Democratic members of Senate Judiciary Committee have called on President Trump to "immediately withdraw" Judge Kavanaugh.
The panel chairman, Chuck Grassley, has left open the possibility that its members may not vote on the nomination by the end of the week.
"It could take place Friday, or it could not," he told reporters in a conference call, reports the Des Moines Register. "That kind of depends on what happens tomorrow."
But Republican Senator Lindsey Graham tweeted: "I have a difficult time believing any person would continue to go to - according to the affidavit - ten parties over a two-year period where women were routinely gang raped and not report it."
What happens on Thursday?
The latest allegation makes Thursday's testimony make or break for the would-be Supreme Court justice.
He will give evidence followed by his first accuser, Christine Blasey Ford, a California psychology professor.
The Senate Judiciary Committee shared Judge Kavanaugh's prepared testimony on the eve of the hearing - before the third allegation emerged.
In his statement, the judge will call sexual assault "horrific", adding: "I have never sexually assaulted anyone - not in high school, not in college, not ever."
"The allegation of misconduct is completely inconsistent with the rest of my life. I have always promoted the equality and dignity of women."