Twitter made an unprecedented move yesterday when it included a fact check under a tweet by President Donald Trump.

Under a tweet where the President claimed mail-in voting will be "substantially fraudulent" and "Mail boxes will be robbed, ballots will be forged & even illegally printed out & fraudulently signed," Twitter added a link to their Events page titled, "Trump makes unsubstantiated claim that mail-in ballots will lead to voter fraud." The page includes links to The Washington Post, NBC, and CNN, all of whom published stories about why the idea that mail-in voting will lead to election fraud is itself fraudulent.

There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent. Mail boxes will be robbed, ballots will be forged & even illegally printed out & fraudulently signed. The Governor of California is sending Ballots to millions of people, anyone…..

— Donald J. Trump (@realDonaldTrump) May 26, 2020

Trump has repeatedly escalated attacks on the idea of mail-in voting, which many states are looking towards as a safe alternative to in-person voting amidst the COVID-19 pandemic. The fact-check marks the first time Twitter has done anything regarding a Trump tweet, which critics of Trump and his tweets have long called for.

In response, Trump tweeted a possible threat to social media platforms that he would "close them down" in the event they "silence conservative voices." This echoes concerns expressed at a White House meeting between Trump and conservative social media presences last year.

Republicans feel that Social Media Platforms totally silence conservatives voices. We will strongly regulate, or close them down, before we can ever allow this to happen. We saw what they attempted to do, and failed, in 2016. We can’t let a more sophisticated version of that….

— Donald J. Trump (@realDonaldTrump) May 27, 2020

Closing Twitter or any social media platform could not be done by the President alone. It would require action by Congress or the Federal Communications Commission.