WASHINGTON (AP) — President Donald Trump, the historically prolific tweeter of political barbs and blasts, 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 has 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.

Tech giants "silence conservative voices," he claimed on Twitter early Wednesday. "We will strongly regulate, or close them down, before we can ever allow this to happen." 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.

Twitter's decision to mark the president's tweets regarding mail-in balloting came as the president was sparking another social media firestorm, continuing to stoke a debunked conspiracy theory accusing MSNBC host Joe Scarborough of killing a former staffer. Prominent Republicans, including Rep. Liz Cheney and Sen. Mitt Romney, urged Trump to drop the attack — which has not been marked with a fact check by the social media company.

Trump and his campaign had lashed out at the company 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.

But some Trump allies, who have alleged bias on the part of tech companies, have questioned whether platforms like Twitter and Facebook should continue to enjoy liability protections as "platforms" under federal law — or be treated more like publishers, which can face lawsuits over content.

The protections have been credited with allowing the unfettered growth of the internet for more than two decades, but now some Trump allies are advocating that social media companies face more scrutiny.

"Big tech gets a huge handout from the federal government," Republican Sen. Josh Hawley told Fox News. "They get this special immunity, this special immunity from suits and from liability that's worth billions of dollars to them every year. Why are they getting subsidized by federal taxpayers to censor conservatives, to censor people critical of China."

Meanwhile, Republicans were turning their fire on one of the Twitter executives responsible for adding the fact checks, Yoel Roth, its head of site integrity. They are pointing to tweets he sent in 2016 and 2017 railing against the president and his allies.

"From their bogus 'fact check' of @realDonaldTrump to their 'head of site integrity' displaying his clear hatred towards Republicans, Twitter's blatant bias has gone too far," tweeted Republican National Chairman Chair Ronna McDaniel.

Former federal judge Michael McConnell, who now directs Stanford Law School's Constitutional Law Center, said Trump lacks the legal power to back up his threat.

"He has no such authority," McConnell said in an email. "He is just venting."

Jack Balkin, a Yale University law professor and First Amendment expert, said any attempt to regulate social media companies for the content on their sites would likely need congressional input and approval — and probably face strong legal challenges.

"The president can't do very much, but that's not the point," he said. "This is an attempt by the president to, as we used to say in basketball, work the refs."

"He's threatening and cajoling with the idea that these folks in their corporate board rooms will think twice about what they're doing, so they won't touch him."

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AP writer David Klepper contributed