The Knight First Amendment Institute, a non-profit set up by the Knight Foundation and Columbia University, is calling on President Trump to unblock users who have been denied access to the @realdonaldtrump account after they criticized him or expressed disagreement.

The institute, in a letter to Trump, warned that the President could face legal action on behalf of two clients they represent.

“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,” the letter stated. It was signed by Jameel Jaffer, the director of the institute; senior litigator Katie Fallow; and senior staff attorney Alex Abdo.

They cited the case of Holly O’Reilly, who tweets under the handle @aynrandpaulryan, who was blocked on May 28 after she posted a GIF captioned, “This is pretty much how the whole world sees you.” In the photo, Pope Francis appears uncomfortable during a meeting with Trump at the Vatican last month.

Also cited was Joseph Papp, who tweets under the handle @joepabike, who discovered he was blocked after he posted on June 4 the message, “Why didn’t you attend your #PittsburghNotParis rally in DC, Sir? #fakeleader.” It was a response to Trump’s tweet of his weekly radio address.

The institute argues that Trump’s Twitter account “is a forum for expression in which you share information and opinions relating to government policy with the public at large, and in which members of the public can engage you, engage one another, and sometimes elicit responses from you.”

Trump has another Twitter account, @POTUS, which he inherited from President Barack Obama and is usually reserved for official announcements or links to significant White House events.

But the Knight Institute argues that Trump’s personal account — where he is prone to express his own, spur-of-the-moment thoughts, is also a public account, even though Trump established the handle when he was a private citizen and then when he was a candidate.

On Tuesday, White House spokesman Sean Spicer was asked about whether Trump’s tweets are also official statements by the President, and he said that they were.

The Knight letter states that “it is plain that our clients were blocked from your account on the basis of their viewpoints. Each of them was blocked soon after having disagreed with or ridiculed you. Other users appear to have been blocked for similar reasons.

“Of course, 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.”

In a statement, Jaffer said that “though the architects of the Constitution surely didn’t contemplate presidential Twitter accounts, they understood that the President must not be allowed to banish views from public discourse simply because he finds them objectionable. Having opened this forum to all comers, the President can’t exclude people from it merely because he dislikes what they’re saying.”