U.S. Senate Judiciary Committee Chairman Chuck Grassley is asking for a criminal investigation into whether Julie Swetnick and her attorney, Michael Avenatti, conspired to provide false statements to Congress and obstructed a congressional investigation during the confirmation process for Supreme Court Justice Brett Kavanaugh.

Grassley wrote in a letter to the Justice Department that Swetnick and Avenatti made serious allegations that required significant resources to investigate. But Grassley says information from media interviews and elsewhere indicates their statements "likely contained materially false claims."

"I don't take lightly making a referral of this nature, but ignoring this behaviour will just invite more of it in the future," Grassley said.

Kavanaugh's confirmation was delayed amid allegations and subsequent investigations. (Michael Reynolds/Reuters)

Avenatti released a sworn statement in which Swetnick said she witnessed Kavanaugh "consistently engage in excessive drinking and inappropriate contact of a sexual nature with women in the early 1980s." Kavanaugh denied the allegations.

Avenatti told The Associated Press he would put his client's credibility up against Grassley's any day: "This is a classic case of be careful what you wish for."

"The only way you test whether my client's allegations were true is if you find out whether they were true or not" through an investigation, Avenatti said.

Kavanaugh's confirmation was delayed and nearly derailed when Dr. Christine Blasey Ford made sexual assault allegations against Kavanaugh. Amid the Judiciary Committee's investigation, Avenatti submitted the sworn statement to the committee from Swetnick.

Grassley said that within hours of Avenatti submitting the statement, Democrats sought to cancel a committee vote on Kavanaugh. He said the committee investigated the claims and diverted significant resources to the effort. He said that Swetnick subsequently contradicted herself in media interviews and there is lack of credible evidence that she ever met or socialized with Kavanaugh. Friends of Kavanaugh also told the committee they had never heard of Swetnick, Grassley said.

"It takes courage to come forward, especially with allegations of sexual misconduct or personal trauma. I'm grateful for those who find that courage," Grassley said in a statement. "But in the heat of partisan moments, some do try to knowingly mislead the committee. That's unfair to my colleagues, the nominees and others providing information who are seeking the truth. It stifles our ability to work on legitimate lines of inquiry."

Avenatti says he is thinking about running for president in 2020. He also represents porn actor Stormy Daniels, who claims she had an affair with U.S. President Donald Trump. Trump denies the allegation.