A federal judge ruled on Wednesday that Donald Trump's use of the Twitter block button violated the First Amendment. The ruling has implications for any government official—federal, state, or local—who uses Twitter or other social media platforms to communicate with the public.
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The main effect of blocking someone is that that person's tweets no longer show up in the blocker's timeline. No one disputes that Trump has the right to do that if he wants. But blocking someone also works in the other direction: if Trump blocks another user, that user can't see Trump's tweets and (as a consequence) can't reply to them. And that, ruled Naomi Buchwald, a New York Federal judge, raises a constitutional problem.
The courts have long held that if the government creates a "public forum"—like a park, lecture hall, or street corner—that it must make the forum available to all speakers, regardless of their viewpoint. A city government can't say that only Republicans, Christians, or vegetarians are allowed to hold rallies in the town square, and it can't blacklist activists with a history of criticizing the mayor.
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If Donald Trump blocks someone, that person loses the opportunity to reply to Trump tweets and have their tweet show up underneath them. They also lose the ability to retweet Trump tweets.
Buchwald's ruling concludes that the "interactive space for replies and retweets" surrounding each Donald Trump tweet should be considered a public forum under First Amendment law. As a result, blocking these users from replying to and retweeting Trump tweets violated the First Amendment.