President Donald Trump’s continued bogus claims that between three and five million illegal votes were cast in the 2016 presidential election have drawn concern and criticism from voter fraud experts who say the allegation hurts the new administration’s credibility and paves the way for severe voter restrictions.

Trump, who first tossed out the baseless allegation following a victory where he won the Electoral College but lost the popular vote, reportedly doubled down on the claim during a meeting Monday with congressional leaders. Asked about the issue at Tuesday’s press briefing, White House press secretary Sean Spicer declined to back down from the assertion, saying, “the president does believe that.”

Trump went a step further on Twitter this morning, tweeting: “I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states, those who are illegal and.... even, those registered to vote who are dead (and many for a long time). Depending on results, we will strengthen up voting procedures!”

Trump’s lie that millions voted illegally came from conspiracy theorists like radio host Alex Jones, whose Infowars website began propagating the false claims shortly after the November 8 election. Right-wing media have been claiming for years that there’s widespread voter fraud despite evidence to the contrary.

But election experts who have studied voter fraud repeated the long-held view that such widespread activity did not occur in this election and would be “impossible” to undertake. Several also urged reporters to continue asking for proof and evidence and make clear there is no basis for such a charge.

“Neither the president nor his press secretary has produced any evidence to back up their fraud claim,” said Bill Schneider, a visiting professor at the UCLA department of communication studies and former senior political analyst for CNN. “The press has to insist that they produce evidence of such a sensational claim. Unless they do, it should be reported as a bogus argument with no proven validity.”

Schneider went on to explain why such fraud claims are inaccurate, calling them “absurd.”

“Every state controls its own voting laws (and in some states, it's done by local communities),” he said via email. “It would be impossible to perpetrate voter fraud on that scale without attracting attention from the authorities.”

Other experts agreed.

Philip B. Stark, a professor of statistics at the University of California, Berkeley, said, “Every investigation of voter fraud that has taken place has found this is in the ones and twos, not hundreds, thousands and certainly not millions. There’s evidence that it’s not true and there is no evidence that it is true. There is research that can be done.”

Rick Hasen, a law professor specializing in election law at the University of California, Irvine School of Law, said, “I don’t think there is any proof whatsoever of illegal voting in the thousands, much less in the millions, much less that affected the outcome in any state. There are safeguards in place to make sure that this doesn’t happen, both before the fact and after the fact. The rates of non-citizen voting are extremely low, despite people looking for it.”

Hasen also urged reporters to continue correcting the record, saying, “It’s important to point out that the claims are false and that they are not backed by any credible evidence.”

He added that Trump and Spicer’s choice to perpetuate the myth “undermines people’s confidence in the electoral process and I think it can provide the justification for restrictive voting rules. Many more legitimate voters are going to be disenfranchised by these rules than illegitimate voters being barred from voting.”

Lorraine Minnite, author of the 2010 book The Myth of Voter Fraud and a Rutgers University professor of political science, said Trump’s claim is “bizarre.” She also offered concern that it could lead to unfair voter restrictions.