WASHINGTON — Lawyers for Twitter users blocked by President Trump after they criticized or mocked him are asking him to reverse the moves, arguing that the Constitution bars him from blocking people on the social media service.

The request raises novel legal issues stemming from Mr. Trump’s use of his Twitter account, @realDonaldTrump, to make statements about public policy. In a letter sent to Mr. Trump on Tuesday, lawyers for several users he has blocked argued that his account was a “public forum” from which the government may not constitutionally exclude people because it disagrees with views they have expressed.

“This Twitter account operates as a ‘designated public forum’ for First Amendment purposes, and accordingly the viewpoint-based blocking of our clients is unconstitutional,” the letter said. “We ask that you unblock them and any others who have been blocked for similar reasons.”