Columbia University’s Knight First Amendment Institute is suing Donald Trump for blocking people on Twitter, claiming that it violates free speech protections. The institute filed suit today on behalf of seven Twitter users who were blocked by the president, which prevents them from seeing or replying to his tweets. It threatened legal action in a letter to Trump in June, and now “asks the court to declare that the viewpoint-based blocking of people from the @realDonaldTrump account is unconstitutional.”

The lawsuit, which was filed in the Southern District of New York, elaborates on the Knight Institute’s earlier letter. It contends that Trump’s Twitter account is a public political forum where citizens have a First Amendment right to speak. Under this theory, blocking users impedes their right to participate in a political conversation and stops them from viewing official government communication. Therefore, if Trump blocks people for criticizing his political viewpoints, he’d be doing the equivalent of kicking them out of a digital town hall.

Trump has definitely used his Twitter account as an official platform. The White House confirmed that his tweets are official statements, and it’s preserving them as public presidential communications. However, it’s much less clear that it counts as a public forum, or that being prevented from viewing or participating in a Twitter thread chills free speech. Users can still view tweets by logging out or creating a new account, and as First Amendment lawyer and blogger Ken White told Vox, a successful lawsuit could make it difficult for any official Twitter account to block trolls or spammers without worrying about legal action.

Nonetheless, the Knight Institute has printed statements from its seven plaintiffs, who say they feel measurably impacted by the block. “My Twitter following is relatively small, but because my tweets show up in the comment threads under the president’s tweets and can be seen by his millions of followers, my replies could gain traction,” says surgery resident Eugene Gu. “Now I have extremely limited access to the public forum where I once could be heard. I feel cut off and as though I’m being treated like an outsider in my own country.”