Newt Gingrich said Tuesday that President-elect Donald Trump’s biggest mistake since the election was his claiming without any evidence that millions of votes were illegally cast on Election Day.

“The president of the United States can’t randomly tweet without having somebody check it out,” Gingrich, a vice chair on Trump’s transition team, told USA Today’s Susan Page during an appearance on the Capital Download video series. “I mean it just— It makes you wonder about whatever else he’s doing. It undermines much more than a single tweet.”

On Sunday, Trump falsely claimed that “millions” of people voted illegally, preventing him from winning the popular vote. A widely-shared article on the conspiracy website InfoWars, whose creator, Alex Jones, Trump has praised in the past, cited a non-existent report to make an identical bogus claim.

Gingrich also recounted to Page that Trump told him last week, apparently without a hint of irony: “This is really a bigger job than I thought.”

Gingrich said he saw that as a “good” reaction from Trump, because he said as president “you have war and peace, you have enormous powers … and it all comes down to the Oval Office and it all comes down to you.”