President Donald Trump can do many things on Twitter. He can call a former FBI director "slippery" and a former president "cheatin'." He can spontaneously SWITCH TO ALL CAPS and claim media organizations are "fake news."

But now there's one thing a federal judge says Trump can't do: Block users because of dissenting political opinions.

U.S. District Judge Naomi Reice Buchwald in New York ruled last Wednesday that Trump violates the First Amendment when he blocks critics on Twitter. The decision is the most prominent among a string of similar cases against public officials and could mark a turning point for constituents who interact with government employees on social media.

"The First Amendment is designed to encourage the greatest amount of freedom in communicating with public officials of all kinds, and that's why this is such an important decision — because it sends a message to all politicians," said Chris Finan, executive director of the National Coalition Against Censorship.

Kerri Kupec, a spokeswoman for the Department of Justice, said in an email after the ruling: "We respectfully disagree with the court's decision and are considering our next steps."

Though the ruling stopped short of ordering Trump to unblock users, it made clear that people have a right to reply directly to politicians' accounts when they are being used as public forums to conduct official business.

But though the courts may be striking up a conversation about what an official's online presence means for free speech, tech companies are unlikely to take sides.

NOT JUST TRUMP

Trump has blocked both celebrities — including author Stephen King and model Chrissy Teigen — and lesser known social media users.

The case decided last week was brought by the Knight First Amendment Institute at Columbia University on behalf of seven critics of Trump whom his account blocked.

When someone like Trump blocks people, it prevents them from seeing his feed and replying to his tweets. It's also possible for Trump to mute accounts, after which he would simply no longer see tweets and replies from those accounts —an action that some say has fewer First Amendment implications.

Trump is far from the only public official who has blocked opponents. After filing a public records request with 22 federal agencies and all 50 U.S. governors last year, news organization ProPublica found the offices that responded had blocked 1,298 social media accounts.

When a Virginia official blocked one Facebook user for 12 hours, a federal judge ruled the move was unconstitutional. The case is under appeal in the 4th U.S. Circuit Court of Appeals.

In Kentucky, a federal judge denied a preliminary injunction to stop the governor from blocking constituents on Twitter and Facebook.

A case in Maryland settled when Republican Gov. Larry Hogan agreed to adopt a new social media policy that would not discriminate against commenters' viewpoints after he blocked several critics.

Though last week's ruling puts officials using social media for policymaking on notice that they can't block people who disagree with them, attorneys said it leaves room to bar commenters who are inhibiting productive discourse.

TWITTER AND THE CONSTITUTION

According to the U.S. Constitution, the president must provide Congress with information about the state of the union. Otherwise, his obligations to communicate with citizens are fairly nebulous.

But First Amendment experts say members of the government cannot impede free speech in public forums or keep people from petitioning their grievances, including on social media.

"If the president doesn't want to hear from his critics, he should stick to press releases," Finan said.

Attorneys warned that if Wednesday's ruling had gone differently, it could have set a precedent for elected officials tempted to rid themselves of their online critics.

WHAT COMES NEXT

Jameel Jaffer, the Knight Institute's executive director, said none of the people his group represents have been unblocked yet.

Twitter probably won't intervene as social media juggernauts have kept out of the issue. The tech company did not respond to requests for comment.

Jaffer said it would be extraordinary if the White House disregarded the court's decision, but says he'll be back in court if it does.

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Villarreal reported from Philadelphia and Ortutay from New York.