But the appeals court disagreed, saying Mr. Trump was clearly acting in a government capacity in his use of Twitter.
“We are not persuaded,” Judge Parker wrote. “We conclude that the evidence of the official nature of the account is overwhelming. We also conclude that once the president has chosen a platform and opened up its interactive space to millions of users and participants, he may not selectively exclude those whose views he disagrees with.”
The ruling upheld a May 2018 decision by a Federal District Court judge that also found Mr. Trump’s practice of blocking his critics from his Twitter account to be unconstitutional. After that ruling, the White House unblocked the specific plaintiffs’ accounts — but not other users who were not involved in the case — while filing an appeal.
Judge Parker was appointed by former President George W. Bush. He was joined in the opinion by Judges Peter Hall, another Bush appointee, and Christopher Droney, an appointee of former President Barack Obama. The district court judge whose earlier ruling the panel affirmed was Naomi Buchwald, a Clinton appointee.
Courts have increasingly been grappling with how to apply the First Amendment, written in the 18th century, to the social-media era. In 2017, for example, the Supreme Court unanimously struck down a North Carolina law that had made it a crime for registered sex offenders to use websites like Facebook.
In January, a panel on the Court of Appeals for the Fourth Circuit, in Richmond, Va., issued a similar ruling in a much smaller-scale case, barring the chairwoman of a board of county supervisors from blocking a critic from a Facebook page she administered. The Knight First Amendment Institute also represented that plaintiff.
In a concurring opinion in the earlier case, Judge Barbara Milano Keenan said the Supreme Court will eventually need to address many difficult issues raised by officials’ use of social-media services. Among others, she questioned whether such companies’ policies of restricting users deemed to use hate speech from their platforms raised a constitutional problem.