Columbia Law School’s Knight First Amendment Institute is asking President Donald Trump to unblock people on Twitter — and threatening him with legal action if he doesn’t comply.

Trump, like many other Twitter users, routinely blocks critics, trolls, and other ne’er-do-wells from following him on the social media platform. But, as president of the United States, Trump is not like any other Twitter users.

The Knight Institute decided to let Trump know in a letter written on behalf of individuals who have been blocked that he could be running afoul of the Constitution.

“We write on behalf of individuals who have been blocked from your most-followed Twitter account, @realDonaldTrump, because they disagreed with, criticized, or mocked you or your actions as president,” the Knight Institute letter reads. “This Twitter account operates as a ‘designated public forum’ for First Amendment purposes, and accordingly the viewpoint-based blocking of our clients is unconstitutional. We ask that you unblock them and any others who have been blocked for similar reasons.”

Several users are cited, including Holly O’Reilly, who goes by the Twitter handle @AynRandPaulRyan. She was blocked after the following Tweet: