The move, which may expose social media companies to liability for what gets posted on their platforms, dramatically escalates a confrontation between Trump and his favored mode of communication, where the president can address his more than 80 million followers directly without relying on news conferences or the traditional media.

Trump and his backers claimed Twitter was suppressing free speech by labeling his tweets, which falsely claimed mail-in ballots would be “substantially fraudulent.” The president typically uses the platform to brag, attack rivals, bolster allies and spread falsehoods.

When a reporter asked at the signing why Trump didn’t simply stop using using the platform, the president retorted, “If we had fair press in this country, I would do that in a heartbeat.”

It wasn’t immediately clear how, if at all, the order can be enforced. The president cannot regulate tech companies without congressional approval and any challenge to their autonomy is sure to end up in court.

“Much as he might wish otherwise, Donald Trump is not the president of Twitter. This order, if issued, would be a blatant and unconstitutional threat to punish social media companies that displease the president,” the American Civil Liberties Union noted on Twitter.

“The president has no authority to rewrite a congressional statute with an executive order imposing a flawed interpretation of Section 230,” the ACLU continued, referring to the section of the Communications Decency Act that shields platforms from being held liable for what users publish on them.

“Ironically, Donald Trump is a big beneficiary of Section 230,” the legal nonprofit continued. “If platforms were not immune under the law, then they would not risk the legal liability that could come with hosting Trump’s lies, defamation, and threats.”

But Trump insisted Thursday that he would go as far as shutting down Twitter if his lawyers found a way. “I’d have to go through a legal process,” he said.

Social media companies have enjoyed legal protections for what gets posted on their platforms and have resisted tampering with even vile falsehoods, including Trump’s aspersions that MSNBC host Joe Scarborough committed murder.

Still, Twitter labeled two of Trump’s tweets with a fact-check warning for the first time on Tuesday after the president’s mail-in fraud claim. Many states have moved to expand vote-by-mail during the coronavirus pandemic, including California, which said this month all registered voters would be sent ballots for the general election.

The move, however, sparked a dramatic outburst from the Oval Office.

Twitter “is now interfering in the 2020 Presidential Election,” Trump wrote. He later said the social media giant was “completely stifling FREE SPEECH, and I, as President, will not allow it to happen!”

Trump doubled down on that criticism Thursday afternoon, tagging Twitter’s head of site integrity Yoel Roth in a separate tweet and deriding him as a “hater.” It’s the second time in as many days the White House has specifically targeted Roth. White House adviser Kellyanne Conway spelled out his Twitter handle on Fox News Wednesday, ominously predicting “he’s about to get a lot more followers.”

Facebook CEO Mark Zuckerberg said he disagreed with Twitter’s policy in an interview with Fox News set to air Thursday.

“I just believe strongly that Facebook shouldn’t be the arbiter of truth of everything that people say online,” Zuckerberg said. “I think in general, private companies probably shouldn’t be — especially these platform companies — shouldn’t be in the position of doing that.”

Twitter CEO Jack Dorsey fired back at the criticism, saying the company would “continue to point out incorrect or disputed information about elections globally. And we will admit to and own any mistakes we make.”

“This does not make us an ‘arbiter of truth,’” Dorsey said. “Our intention is to connect the dots of conflicting statements and show the information in dispute so people can judge for themselves. More transparency from us is critical so folks can clearly see the why behind our actions.”

Lydia O’Connor and Ryan Grenoble contributed reporting.