A classmate of Supreme Court nominee Brett Kavanaugh has accused the judge of lying under oath about "heavy" alcoholic drinking.
Charles Ludington said he was "deeply troubled" by Judge Kavanaugh's "blatant mischaracterisation". The judge denies drinking to the point of memory loss.
The matter forms part of an FBI inquiry into the judge, as do allegations of sexual assault, which he also denies.
President Donald Trump said he had not limited the inquiry's scope.
The FBI inquiry has delayed a final vote on Judge Kavanaugh, who if confirmed is likely to make America's highest court more conservative.
Rachel Mitchell, the Arizona prosecutor hired by Republicans to question one of the judge's accusers, Prof Christine Blasey Ford, during hearings last week, gave her memo to senators on Sunday.
In it, she questioned Prof Ford's credibility, calling her case weak and saying she did not think "that a reasonable prosecutor would bring this case based on the evidence before the Committee".
What did the classmate say about Judge Kavanaugh?
Prof Ludington, who teaches at North Carolina State University, said he had seen Judge Kavanaugh slurring his words and staggering after excessive alcohol consumption while at Yale.
The professor also said Judge Kavanaugh "was often belligerent and aggressive" when drunk.
"I can unequivocally say that in denying the possibility that he ever blacked out from drinking, and in downplaying the degree and frequency of his drinking, Brett has not told the truth."
He noted that it was not Judge Kavanaugh's drinking habits in college that worried him - it was the fact that he made questionable statements under oath.
"If he lied about his past actions on national television, and more especially while speaking under oath in front of the United States Senate, I believe those lies should have consequences," Prof Ludington said.
The professor plans to speak to the FBI on Monday, according to the New York Times.
His statement contradicts another Yale classmate - former NBA player Chris Dudley - who told the Washington Post he "never, ever saw Brett Kavanaugh black out" from alcohol consumption.
When asked about Judge Kavanaugh's drinking habits during a news conference on Monday, Mr Trump said he thought his answers during the hearing were "excellent".
"He had a little bit of difficulty, he talked about things that happened when he drank," Mr Trump said. "This is not a man who said that he was perfect with respect to alcohol."
The president's son, Donald Trump Jr, has said the #MeToo movement makes him more worried about his sons' future than his daughters'.
"I've got boys. I've got girls," Mr Trump Jr told Daily Mail TV. "And when I see what's going on it's scary."
He added that sexual assault victims should feel "able to come out and be believed" with the caveat: "There has to be some credibility behind it."
How teens are talking about sex in the #MeToo age
Trump's line of defence
Analysis by Anthony Zurcher, BBC News Washington
There was considerable confusion last Friday, when Donald Trump ordered a reopened investigation into the "credible allegations" against Brett Kavanaugh.
In comments on Monday, the president did little to clear things up.
What Mr Trump was clear about, however, was that while Judge Kavanaugh may have had "difficulty as a young man" with alcohol, that alone shouldn't disqualify him from the high court.
Nobody, the president said, has questioned how the judge has behaved "in the last 20 years".
Mr Trump's answer was a direct response to Democrats who have suggested that Judge Kavanaugh was less than forthcoming in his testimony under oath about his alcohol use while in high school and college.
With the FBI delving into Judge Kavanaugh's background - including receiving a statement from a Yale classmate who says he misrepresented the extent of his alcohol use - there's the chance more details will emerge about Judge Kavanaugh's past actions.
With his comments, the president is constructing a line of defence to keep undecided Senate Republicans - the votes necessary to confirm Judge Kavanaugh - from turning against the nominee when the final FBI report comes in.
What is the FBI investigating?
Judge Kavanaugh's behaviour at Yale is under the spotlight, with Prof Ludington and Mr Dudley among several of his classmates who have recently spoken out.
The FBI inquiry is expected to be completed within a week.
In terms of the sexual assault allegations, US media report that the FBI spoke on Sunday to another of the accusers, Deborah Ramirez.
Opposition Democrats have accused the administration of trying to limit the scope of the inquiry, amid media reports that Judge Kavanaugh's third accuser - Julie Swetnick - would not be interviewed.
Mr Trump has denied imposing any restrictions, saying he wanted the FBI "to interview whoever they deem appropriate".
Wow! Just starting to hear the Democrats, who are only thinking Obstruct and Delay, are starting to put out the word that the “time” and “scope” of FBI looking into Judge Kavanaugh and witnesses is not enough. Hello! For them, it will never be enough - stay tuned and watch!— Donald J. Trump (@realDonaldTrump) September 30, 2018
Despite this, NBC News quoted a White House official as saying restrictions remained in place, adding that as the FBI was carrying out a background - not a criminal - investigation, the White House decided the parameters of the investigation.
The most senior Democrat on the Senate Judiciary Committee, Dianne Feinstein, has sent a letter urging the White House to release the written directive sent by President Trump to the FBI launching the investigation.
Although the committee on Friday approved Judge Kavanaugh's nomination, it was conditional on a new inquiry into what it said were the "credible allegations" facing him.
As the investigation is not a criminal one, the FBI will not say whether it believes the allegations are true.
Who are Judge Kavanaugh's accusers?
Psychology professor Christine Blasey Ford was the first woman to come forward. She testified at a hearing last week that Judge Kavanaugh tried to remove her clothing, pinned her to a bed and covered her mouth at a house party in 1982, when she was 15 and he was 17.
In response to her testimony, Judge Kavanaugh said he had never assaulted her or anyone else. He accused Democrats of politicising the process and harming his family and good name.
Deborah Ramirez attended Yale at the same time as Judge Kavanaugh. She says he exposed his genitals to her during a college drinking game.
Julie Swetnick says she went to house parties attended by Judge Kavanaugh in the early 1980s, where she said he and his friends had tried to "spike" girls' drinks.
He has denied all the accusations.