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 engaging with his account — was declared unconstitutional by a federal judge in Manhattan.Judge Naomi Reice Buchwald, addressing a novel issue 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.