WASHINGTON (AP) — President Donald Trump threatened social media companies with new regulation or even shuttering on Wednesday after Twitter added fact checks to two of his tweets. He turned to his Twitter account – where else? — to tweet his threats.

The president can’t unilaterally regulate or close the companies, and any effort would likely require action by Congress. His administration shelved a proposed executive order empowering the Federal Communications Commission to regulate technology companies, citing concerns it wouldn’t pass legal muster. But that didn’t stop Trump from angrily issuing strong warnings.

Trump, the historically prolific tweeter of political barbs and blasts, claimed on Twitter early Wednesday that tech giants “silence conservative voices.”

“We will strongly regulate, or close them down, before we can ever allow this to happen,” he tweeted. Later, also on Twitter, he threatened, “Big Action to follow.”

He repeated his unsubstantiated claim — which sparked his latest showdown with Silicon Valley — that expanding mail-in voting “would be a free for all on cheating, forgery and the theft of Ballots.”

There was no immediate reaction from Twitter or other social media companies to the president’s threats.

Trump and his campaign had lashed out Tuesday after Twitter added a warning phrase to two Trump tweets that called mail-in ballots “fraudulent” and predicted that “mail boxes will be robbed,” among other things. Under the tweets, there is now a link reading “Get the facts about mail-in ballots” that guides users to a Twitter “moments” page with fact checks and news stories about Trump’s unsubstantiated claims.

Trump replied on Twitter, accusing the platform of “interfering in the 2020 Presidential Election” and insisting that “as president, I will not allow this to happen.” His 2020 campaign manager, Brad Parscale, said Twitter’s “clear political bias” had led the campaign to pull “all our advertising from Twitter months ago.” Twitter has banned all political advertising since last November.

Trump did not explain his threat Wednesday, and the call to expand regulation appeared to fly in the face of long-held conservative principles on deregulation.