President Trump's first White House meeting with congressional leaders was, by all accounts, a cordial affair.

But Trump apparently couldn't resist taking a jab across the aisle, telling the gathering of bipartisan leaders that he'd lost the popular vote only because of rampant voter fraud by "illegals," according to a pair of sources familiar with the conversation.

"He said 3 to 5 million 'illegals' voted so that's why he lost popular vote," said a Democratic aide.

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"That's exactly what he said," said a second source familiar with the exchange.

Trump secured his presidential victory by winning the Electoral College vote, 304-227. But his opponent, Democrat Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE, won the popular ballot by almost 3 million votes.

The dynamics have undermined Trump's claims that the election gave him a mandate to move his policy agenda, which includes undoing large parts of former President Obama's legislative legacy. And Trump has pushed back by publicly claiming that he would have won the popular vote if "illegals" were disqualified.

“In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally,” Trump tweeted on Nov. 27.

That verdict has been quashed by state officials and independent fact-checkers. PolitiFact put his charge in the "pants on fire" category.

But Trump's amplification of that claim on Monday suggests the issue remains a sore spot, even after his swearing in.

The White House did not immediately respond to a request for comment Monday.