Trump Repeats Unsubstantiated Claim About Voter Fraud During Election Trump made the claim to Congressional leaders.

 -- Donald Trump has repeated the unsubstantiated claim that millions of people voted illegally in the 2016 election, a claim that he used after the election to justify his loss of the popular vote, Democratic and Republican sources told ABC News.

"Serious" voter fraud and a "rigged" system were themes of Trump's campaign, both in the primaries and general election, assertions that carried past Election Day. Trump, who has made a series of unsubstantiated or false claims, won the Electoral College with 306 votes (2 eventually defected), but lost the popular vote by nearly 3 million ballots.

During the meeting with Congressional leadership, Trump said "3 to 5 million illegals" voted, according to two Democratic aides. But a Republican aide said the comments were made in jest.

"He was giving them a hard time," the Republican aide said, a characterization a House Democratic aide disputed.

ABC News contacted several White House officials who have not responded to a request for comment.

In the wake of the Nov. 8 election, Trump tweeted that he won the Electoral College in a "landslide" and claimed that millions had voted illegally.

He also claimed on Nov. 27, again without offering evidence that there was "serious" voter fraud in three states.

During the election, Trump repeatedly made the claim that the system was "rigged," despite little evidence of voter fraud over the past several elections.

An ABC News fact-check found the claim about serious voter fraud to be false.

Trump transition spokesman Jason Miller cited two studies as evidence for the then-president-elect's claim, but the authors of both of those studies disputed his characterizations.

A spokeswoman for the National Association of Secretaries of State, Kay Stimson, said in November the organization has "no information that can help to explain what sources or information are behind the basis of the tweets" by Trump.