Former House Speaker Newt Gingrich (R) said tweeting a lie about voter fraud was the worst thing President-elect Donald Trump has done since winning the election.

While speaking Tuesday on USA Today’s “Capital Download,” Gingrich said presidents “can’t randomly tweet without having somebody check it out.”

“It makes you wonder about whatever else he’s doing,” Gingrich said. “It undermines much more than just a single tweet, so I’d say that’s probably the biggest single thing he’s done wrong.”

In a tweet Sunday afternoon, Trump alleged there were “millions of people who voted illegally” in this year’s presidential election. That claim has been rejected by a number of states, election law experts and lawmakers on both sides of the aisle.

Gingrich has been a long-time supporter of the president-elect, but he hasn’t shied away from pointing out Trump’s mistakes. Gingrich called Trump’s racist comments about federal Judge Gonzalo Curiel “inexcusable,” saying in June it was “one of the worst mistakes Trump has made.”

Gingrich has also repudiated Trump’s economic plan and struggled to defend Trump’s tendency to make false remarks.

Watch Gingrich’s remarks on Trump’s voter fraud lie above.