The Knight First Amendment Institute, a nonprofit advocacy group based at Columbia University, threatened to take legal action against President Trump if he does not unblock critics on Twitter.

The demand, made in a letter to President Trump, was sent on behalf of Holly O'Reilly and Joseph M. Papp, two Twitter users who were blocked by the president's account after criticizing him on the social media network.

The letter argues that President Trump's Twitter account constitutes a "designated public forum" and is subject to the protections of the First Amendment. According to precedent established by the Supreme Court, designated public forums are places "set aside by government for expressive activities" including "parks, sidewalks and areas that have been traditionally open to political speech and debate."

Related Training: Access Denied: Your Rights When Government Shuts Media Out

"This is a context in which the Constitution precludes the President from making up his own rules," said Jameel Jaffer, the Knight Institute’s executive director, in a statement accompanying the demand.

"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."

In a brief interview, Jaffer said The Knight First Amendment Institute may consider legal action if the Trump administration doesn't unblock the president's critics.

"You can, in theory, sue President Trump," Jaffer said. "We're hoping the president will unblock our clients and others that have criticized him."

Here's the letter: