President Donald Trump regularly blocks people from following him on his @realDonaldTrump feed on Twitter.

But now, two Twitter users who were blocked by Trump after they criticized the president are claiming that their exclusion from being one of Trump's 31.7 million followers amounts to a First Amendment breach of their constitutional rights. Because of that, the Knight First Amendment Institute at Columbia University is asking Trump to unblock them and anybody else he has blocked on Twitter.

It's a novel request and a unique legal theory. The institute calls Trump's account a "designated public forum." Because of that, the institution tells the president, "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 metaphysical public forum

The letter also makes the same request for the president's @POTUS account.

According to the institute:

When the government makes a space available to the public at large for the purpose of expressive activity, it creates a public forum from which it may not constitutionally exclude individuals on the basis of viewpoint. This is true even if the space in question is "metaphysical" rather than physical; even if the space is privately rather than publicly owned; and "even when the limited public forum is one of [the government's] own creation." The government may impose reasonable time, place, and manner restrictions in a designated public forum, but it may not exclude people simply because it disagrees with them.

The institute said it might sue if its clients' Twitter feeds remain blocked. According to the micro-blogging service, blocking "is a feature that helps you control how you interact with other accounts on Twitter. This feature helps users in restricting specific accounts from contacting them, seeing their Tweets, and following them."

The issue comes as the courts are considering Trump's tweets on immigration as they weigh the constitutionality of his immigration executive order. Also, the development highlights that society has reached the point when public officials, from the presidency on down, are harnessing social media as a means of political discourse.

"When new communications platforms are developed, core First Amendment principles cannot be left behind," Katie Fallow, a senior litigator at the Knight Institute, said in a statement. "The First Amendment disallows the president from blocking critics on Twitter just as it disallows mayors from ejecting critics from town halls."

The institute says that Twitter user Holly O'Reilly, @AynRandPaulRyan, was blocked May 28 after posting a GIF with the caption: "This is pretty much how the whole world sees you." The GIF shows Pope Francis appearing "incredulous and uncomfortable" while meeting Trump.

The other blocked user is Joseph M. Papp, @joepabike, who learned he was blocked on June 4 for criticizing Trump while using the tag #fakeleader.