Sean Spicer. Screenshot/White House Reporters grilled White House press secretary Sean Spicer during Tuesday's press briefing about President Donald Trump's renewal of his false claim that millions of people voted illegally in the November election.

Trump has said that he would have won the popular vote, which he lost by nearly 3 million votes to Democratic nominee Hillary Clinton, had millions of people not voted illegally — a claim unsupported by evidence and judged untrue by numerous fact-checkers and voting officials.

Trump brought up the claim during a Monday night meeting with congressional leaders, to the dismay of several Republicans.

"The Truth About Voter Fraud," a report written by experts at the Brennan Center for Justice, found that the rate of voter fraud fell between 0.00004% and 0.0009%.

Asked on Tuesday by several reporters about the false claims, Spicer said the president stands by his belief that millions voted illegally, saying it was based on studies and evidence he's been presented.

Spicer insisted that Trump is "very comfortable with his win," which he secured with more than 300 electoral votes. The press secretary seemed to dismiss Trump's Monday remarks as a simple "discussion with some folks."

It's "a longstanding belief he's maintained," Spicer said.

Pressed to provide any of the studies or evidence he was speaking of, Spicer said there was a 2008 study from the Pew Research Center that showed that 14% of people who voted were not citizens.

The study that Trump frequently mentioned during the campaign, however, was not from 2008, but 2012. That Pew report said 24 million voter registrations were no longer valid or were significantly inaccurate, more than 1.8 million dead people were listed as voters, and about 2.8 million people were registered in more than one state. The report did not claim that those people voted illegally in an election, as it was written as an argument for modernizing the US voting system.

Spicer was more likely referencing a 2014 study by Jesse Richman and David Earnest that found that more than 14% of noncitizens in 2008 and 2010 "indicated that they were registered to vote." But the group that designed the study said in 2014 that the sample was so small that the results could be incorrect.

Spicer was asked if the administration would launch a voter fraud investigation, to which he said, "Maybe we will."

He later said there is no investigation at the moment, but that it would be possible to launch one.

Asked if he personally believed that millions voted illegally, Spicer avoided the question.