Attorneys for the U.S. Department of Justice said a preliminary injunction to force President Donald Trump to unblock a group of plaintiffs on Twitter would “send the First Amendment deep into uncharted waters,” according to a letter filed Aug. 11.

The group brought suit against the president last month, in Knight First Amendment Institute at Columbia University v. Trump, 17-cv-5205, arguing that his decision to block them on the social media platform represented a First Amendment violation.

Trump uses Twitter to make official government pronouncements and take official government actions, plaintiffs argue. In blocking their individual accounts, the president has imposed an unconstitutional viewpoint-based restriction to what amounts to an open public forum.