WASHINGTON — President Trump used his first official meeting with congressional leaders on Monday to falsely claim that millions of unauthorized immigrants had robbed him of a popular vote majority, a return to his obsession with the election’s results even as he seeks support for his legislative agenda.

The claim, which he has made before on Twitter, has been judged untrue by numerous fact-checkers. The new president’s willingness to bring it up at a White House reception in the State Dining Room is an indication that he continues to dwell on the implications of his popular vote loss even after assuming power.

Mr. Trump appears to remain concerned that the public will view his victory — and his entire presidency — as illegitimate if he does not repeatedly challenge the idea that Americans were deeply divided about sending him to the White House to succeed President Barack Obama.

Mr. Trump received 304 electoral votes to capture the White House, but he fell almost three million votes short of Hillary Clinton in the popular vote. That reality appears to have bothered him since Election Day, prompting him to repeatedly complain that adversaries were trying to undermine him.

Moving into the White House appears not to have tempered that anxiety. Several people familiar with the closed-door meeting Monday night, who asked to remain anonymous in discussing a private conversation, said Mr. Trump used the opportunity to brag about his victory.

As part of that conversation, Mr. Trump asserted that between three million and five million unauthorized immigrants voted for Mrs. Clinton. That is similar to a Twitter message he posted in late November that said he would have won the popular vote “if you deduct the millions of people who voted illegally.”

Voting officials across the country have said there is virtually no evidence of people voting illegally, and certainly not millions of them. White House officials did not respond to requests for a comment on Mr. Trump’s discussion of the issue.

Representative Steny H. Hoyer, Democrat of Maryland, who attended the meeting, said that Mr. Trump also talked about the size of the crowd for his Inaugural Address.

“It was a huge crowd, a magnificent crowd. I haven’t seen such a crowd as big as this,”…