President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE’s claims that millions of people voted illegally in the 2016 election “undermines the idea of an election” and upends the U.S. Constitution, a Republican lawmaker said Friday.

“This basically undermines the idea of an election. This undermines the Constitution,” Rep. Adam Kinzinger Adam Daniel KinzingerFox News reporter defends confirming Atlantic piece despite Trump backlash: 'I feel very confident' GOP lawmaker defends Fox reporter after Trump calls for her firing Lindsey Graham: 'QAnon is bats--- crazy' MORE (R-Ill.) said on CNN’s “New Day.”

“If you really believe there were 3 to 5 million illegal votes, please share that proof — show us how,” he said.

Trump told congressional leaders earlier this week that he would have won the popular vote if it weren’t for fraudulent voting by “illegals.” He followed up on the comments Wednesday, tweeting that he is planning to order an investigation into voter fraud in the 2016 election.

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By not showing evidence to back up his claim, Kinzinger said, Trump is playing into citizens' personal biases, causing Americans to question the legitimacy of the electoral system.

“When you just attack it and say, ‘Well, I’ll show you proof eventually,' or you say, ‘Hey, look, just trust me on this one,’ you get some people that believe it, because you have confirmation bias on either way,” he said. “Some people that believe it because it seems right, and some people that don’t.”

Kinzinger compared Trump’s comments to Russia's alleged efforts to interfere in the election, saying that both cases ultimately target the legitimacy of the country’s basic principles of government.

The president was expected to sign an executive order launching the investigation on Thursday, but postponed the measure because he “got jammed up on some meetings that needed to occur," White House press secretary Sean Spicer said.

The executive order is now expected to be signed either Friday or Saturday.