The US Department of Justice, which represented the President in the case, argued Mr Trump's own First Amendment rights allowed him to block people with whom he did not wish to interact.
Judge Naomi Reice Buchwald of the US District Court for the Southern District of NY sided with the argument made by the Knight First Amendment Institute at Columbia University that the president's Twitter feed constitutes a "designated public forum", and that Trump therefore can't block users based on their political views.
"We thought it was a very important precedent to establish that the accounts of public officials on social media do operate as public forums where people's First Amendment rights are protected, which enables them to contribute to the dialogue about policy from the local level up to the national level and prevents public officials, effectively, from creating echo chambers where you block out dissent", Knight Institute staff attorney Carrie DeCell said. She said she assumed Trump or his social media director Dan Scavino, who also was a defendant in the case, would unblock the users in light of her decision.
The ruling on the case, filed by the Knight First Amendment Institute at Columbia University, is a declaratory judgment.
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Buchwald said Trump could "mute" users, meaning he would not see their tweets while they could still respond to his, without violating their free speech rights. He had already amassed 20.5 million followers and had posted 34,300 tweets before he took office in January 2017. Muted users could still appear for others to see under Trump's tweets, however.
The White House did not immediately have a comment on the ruling.
Judge Buchwald said she wouldn't order Mr. Trump and Mr. Scavino to change their behavior and didn't want to get into a separation of powers battle.
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Dissenting view: National Review's David French argues against the ruling, writing that because "Donald Trump's Twitter feed isn't a government-controlled forum", the First Amendment protections for public forums shouldn't apply.
The White House directed queries to the Department of Justice, where a spokeswoman said in a statement, "We respectfully disagree with the court's decision and are considering our next steps". And that, ruled Naomi Buchwald, a New York Federal judge, raises a constitutional problem.
All Pres. Trump's many tweets come from his trusty iPhone, and he's not shy about blocking people who use this social network platform to respond to his comments.
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And because of Trump's actions, the court concludes that the use of the account "is governmental in nature".
Blocking users on the basis of political speech is a violation of their free-speech rights, the court said. In last year's ruling about Facebook blocking, the judge held that public officials could do a certain amount of content moderation, provided that it was done in a content-neutral manner.
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