President Trump has blocked a lot of people on Twitter. And now, some are arguing that's a violation of their First Amendment rights.

In a letter to Trump on Tuesday, lawyers from the Knight First Amendment Institute at Columbia University called on the president to unblock people on Twitter (TWTR). The group is representing two Twitter users who were blocked by the president after they tweeted critical statements to him.

The lawyers argue that Trump can't exclude people from engaging with him on Twitter based on their viewpoints.

"Your Twitter account is a designated public forum for essentially the same reasons that open city council meetings and school board meetings are," the lawyers wrote in the letter.

The letter is directed at the @RealDonaldTrump account, but lawyers say it applies to the @POTUS account as well.

When someone is blocked on Twitter, they are unable to follow the account, view the account's tweets when logged in to the service, or view tweets the account has liked.

Related: Trump appears to take his cues from Fox News in tweets on London attack

The letter raises interesting questions about how government social media accounts should be treated. The lawyers aren't saying all Twitter blocking violates the First Amendment, but if government officials use Twitter in an official capacity, they shouldn't be able to block people for expressing an opinion.

"While [the letter] relates to our most prominent Twitter user, the principles we seek to vindicate apply to all public officials and public entities that use social media to conduct government business and allow the public to participate," Katie Fallow, senior attorney at the Knight First Amendment Institute, told CNN Tech.

Nearly all high level public officials use Twitter -- many of them to engage in official business.

Courts have previously said public social media accounts used as public forums should not censor opinions. In Davison v. Loudoun County Board of Supervisors, the plaintiff argued that deleting a post on the Facebook page of a County Supervisor violated the plaintiff's First Amendment rights. The court agreed, saying the county can't discriminate or block people based on their views.

On Tuesday, press secretary Sean Spicer said Trump's tweets are considered official White House statements.

If Trump doesn't unblock Twitter users, Fallow said the Knight First Amendment Institute would consider a lawsuit.