Apart from the man himself, perhaps nothing has defined President Trump’s political persona more than Twitter.
But on Wednesday, one of Mr. Trump’s Twitter habits — his practice of blocking critics on the service, preventing them from appealing with his account — was declared unconstitutional by a federal judge in Manhattan.
Judge Naomi Reice Buchwald, addressing a novel problem about how the Constitution applies to social media platforms and public officials, found that the president’s Twitter feed is a public forum. As a result, she ruled that when Mr. Trump or an aide blocked seven plaintiffs from viewing and replying to his posts, he violated the First Amendment.
If the principle undergirding Wednesday’s ruling in Federal District Court stands, it is likely to have implications far beyond Mr. Trump’s feed and its 52 million followers, said Jameel Jaffer, the Knight First Amendment Institute’s executive director and the counsel for the plaintiffs. Public officials throughout the country, from local politicians to governors and members of Congress, regularly use social media platforms like Twitter and Facebook to interact with the public about government business.
“This ruling should put them on notice, and if they censor critics from social media accounts utilized for official purposes, they run the risk that someone will sue them and win,” he said of public officials.
Asked whether the administration would unblock the users or appeal the ruling, Kerri Kupec, a spokeswoman for the Justice Department’s civil division, demurred from making any particular pronouncement. “We respectfully disagree with the court’s decision and are considering our next steps,” she said.
In her ruling, Judge Buchwald said Mr. Trump and Dan Scavino, the White House social media director, “exert governmental control over certain aspects of the @realDonaldTrump account.”
But she did not issue an injunction ordering Mr. Trump or Mr. Scavino to unblock the users, a nod to the separation-of-powers sensitivities of a judge’s ordering a president to do something. Rather, her ruling easily declared what the Constitution requires, with the expectation that the White House would comply.
“Because no government official is above the law and because all government officials are presumed to follow the law once the judiciary has said what the law is, we must assume that the president and Scavino will remedy the blocking we have held to be unconstitutional,” she wrote in her ruling.
The plaintiffs had argued that Mr. Trump’s Twitter feed is an official government account and that blocking users from following it is a violation of their First Amendment rights.
In June, the plaintiffs had sent a letter to the White House asking to be unblocked. When that went ignored, they sued.
Their complaint argued that Mr. Trump’s feed amounted to a “digital town hall” where not only did the president and his aides communicate information but members of the public — by replying to Mr. Trump’s tweets and others who responded to him — exchanged views with one another. By blocking particular people from viewing or replying to message chains because they had expressed views he did not like, it argued, Mr. Trump had violated their First Amendment rights.
Judge Buchwald, who was appointed to the federal bench in 1999 by President Bill Clinton, agreed.
“The viewpoint-based exclusion of the individual plaintiffs from that designated public forum is proscribed by the First Amendment and cannot be justified by the president’s personal First Amendment interests,” she wrote.
The ruling also rejected the government’s claim that Mr. Trump operates the account merely in a personal capacity, concluding that he “uses the account to take actions that can be taken only by the president as president.”
Judge Buchwald, however, did dismiss Sarah Huckabee Sanders, the White House press secretary, and Hope Hicks, the former communications director, as defendants because Ms. Sanders does not have access to the account and Ms. Hicks no longer works in the administration.
Mr. Jaffer said Mr. Trump or Mr. Scavino should simply unblock the users. If they do not, the next step could be to seek an injunction. (Judge Buchwald mentioned that possibility, writing that “injunctive relief directed at Scavino” was one of the options that “remain available.”)
“The position the Trump administration is taking is that the president is entitled to block people, and that the court lacks the capability to order him to do otherwise,” Mr. Jaffer said. “The right thing for the president and his social media director to do would be to log into the president’s account and unblock everyone who has been blocked on the basis of viewpoint.”
There have been other legal cases involving public officials and their social media activity. In April, Gov. Larry Hogan of Maryland settled a lawsuit filed by the American Civil Liberties Union of Maryland and several other plaintiffs who accused him of censorship on government-operated Facebook pages. A similar federal lawsuit filed against Gov. Paul R. LePage of Maine is awaiting a ruling.
In a March hearing, Judge Buchwald had suggested that Mr. Trump has the right to ignore whomever he pleases on Twitter, and that the “Mute” feature is a constitutional option for doing so. Utilizing that feature would hide selected users’ posts from Mr. Trump’s view.
She urged the two parties to consider that probabilities as a way to settle the matter. But the president’s preference for blocking — which renders Twitter users unable to interact directly with his posts, or with other users in the space beneath those posts — amounts to barring Americans from a public forum, in the judge’s view.
Indeed, many of the tweets for which users were blocked were attempts to join the straggle conversations that take place around the president’s feed.
Nick Pappas, a comic and writer, was blocked after replying to the president in June. Mr. Trump had tweeted, “The Justice Dept. should ask for an expedited hearing of the watered down Travel Ban before the Supreme Court – & seek much tougher version!” Mr. Pappas replied: “Trump is right. The government should protect the people. That’s why the courts are protecting us from him.”
Another plaintiff, Rebecca Buckwalter, who is now an editor for the left-leaning website The Daily Kos, was blocked for replying to a June tweet from Mr. Trump that read, “Sorry folks, but if I would have relied on the Fake News of CNN, NBC, ABC, CBS, washpost or nytimes, I would have had ZERO chance winning WH.”
“To be fair you didn’t win the WH: Russia won it for you,” Ms. Buckwalter tweeted, shortly before being blocked. After Wednesday’s ruling, she took to Twitter again: “I sued the President, and I won.”