A legal group has asked Donald Trump to unblock users who have offended him on Twitter, saying that the blocks violate the First Amendment. In a letter, the Knight First Amendment Institute at Columbia University argued that Trump’s Twitter account was a public forum, and banning users from viewing or engaging with his tweets suppresses free speech. “The President must not be allowed to banish views from public discourse simply because he finds them objectionable,” writes Knight Institute executive director Jameel Jaffer. “Having opened this forum to all comers, the President can’t exclude people from it merely because he dislikes what they’re saying.”

The Knight Institute letter cites the cases of Holly O’Reilly and Joseph M. Papp, two Twitter users who were blocked from Trump’s @RealDonaldTrump account after mocking or criticizing the president. They’re far from the only people who have provoked the same reaction — others can be found in the #BlockedByTrump hashtag. Blocked users can’t see Trump’s tweets or reply to them while logged in, which the letter says “suppresses speech in a number of ways.” They compare the Twitter account to city council meeting or other public forum. “It is easy to understand why you and your advisers might have found our clients’ posts to be disagreeable. Even if the posts were scornful and acerbic, however, they were protected by the First Amendment.”

Trump — like many Twitter users — has been blocking people for years, but the act took on new meaning after his election. The Knight Institute letter refers to both @RealDonaldTrump and the lesser-used @POTUS accounts, both of which serve as platforms for Trump to make official statements — White House press secretary Sean Spicer said today that Trump’s tweets were “official statements by the president of the United States.”

It’s easy for users to log out and view the tweets, but can the president intentionally try to prevent someone from seeing public statements? This is untested legal territory, and a different (and weirder) question from the usual debates over Twitter free speech, which have previously covered account suspensions and shared blocklists. If the administration doesn’t address the question, the Knight Institute says it should prepare to “face legal action to protect the First Amendment rights of the blocked individuals.”