In September 2018, President Donald Trump won an important First Amendment victory. By a 3-0 vote, the U.S. Court of Appeals for the 6th Circuit ruled that neither Trump nor his campaign could be held liable for injuries suffered by protesters at the hands of Trump supporters during a March 2016 rally even though the then-candidate had yelled to “get ’em out of here.”

The court’s reasoning was based on the straightforward application of free-speech principles: Trump had not advocated violence at the Kentucky rally (in fact, he had also said “don’t hurt ’em”), and therefore he couldn’t be successfully sued even though some people were roughed up.

The decision may have been a disappointment for those who thought Trump should be held accountable for his careless words. But under the First Amendment, political speech receives the highest level of protection except in the most extreme circumstances.

Now, though, those principles are in danger. Based on a similar set of facts, the 5th Circuit ruled recently that a police officer in Baton Rouge, Louisiana, can sue Black Lives Matter activist DeRay Mckesson for injuries he received at the hands of a rock-throwing protester — even though, as The Washington Post reported, “Mckesson did not throw the rock or tell anyone else to throw it.”

Mckesson had organized the 2016 demonstration following the police killing of a black man named Alton Sterling. The police officer, whose identity has not been revealed, claims that Mckesson acted negligently by not foreseeing that the demonstration could become violent.

“The goal of lawsuits like these is to prevent people from showing up at a protest out of the fear that they might be held responsible if anything happens,” Mckesson said in a statement released by the ACLU, which earlier this month asked the Supreme Court to take up the case. “If this precedent lasts, it could make organizers all across the country responsible for all types of things they have no control over, such as random people coming into a protest and causing problems. We can’t let that happen.”

The Supreme Court precedent that protected Trump and that, by all rights, should protect Mckesson is NAACP v. Claiborne Hardware Co., a case decided in 1982. The NAACP in 1966 called for a boycott of white-owned businesses in Claiborne County, Mississippi — a nonviolent form of protest that nevertheless led to some acts of violence. In 1969, several of the business owners sued the NAACP and were successful in the state courts. But the Supreme Court ruled unanimously that the NAACP could not be held liable for engaging in nonviolent free-speech activities regardless of actions taken by people not under the organization’s control.

Although the cases against Trump and Mckesson, like the case against NAACP, were for alleged civil offenses, it’s also worth noting the high bar the Supreme Court has set for incitement to violence in criminal cases. In the landmark 1969 case of Brandenburg v. Ohio, the court threw out the conviction of Ku Klux Klan leader Clarence Brandenburg because the threats he made at a rally against African Americans and Jews were non-specific and would not result in imminent violence.

In their petition to the Supreme Court, the ACLU’s lawyers argue that allowing the lawsuit against Mckesson to proceed would have a chilling effect on anyone who might wish to organize a nonviolent protest.

“Given the regularity with which violence and First Amendment activity co-occur and the vagaries of state law liability rules,” the lawyers wrote, “only the most intrepid citizens would exercise their rights and risk ruinous liability if they could be held liable for the wrongful acts of others.”

Ben Wizner, director of the ACLU’s Speech, Privacy and Technology Project, put it this way in the Washington Post story: “If this is allowed to stand, anybody can show up and throw a rock at a protest to bankrupt a movement they disagree with. People know when they step into the street that they might have to spend some hours in jail or pay a fine. But if they might have to pay a multi-million dollar civil judgment — that’s something they’re not prepared for, and can’t possibly be expected to prepare for.”

The case should prove to be a telling indicator of where the Supreme Court stands on free speech now that it has swung sharply to the right. Only four of the nine justices are needed take up the case.

Traditionally, even the court’s more conservative members have proved to be staunch advocates of the First Amendment. But if Trump’s justices, Neil Gorsuch and Brett Kavanaugh, were to break with that tradition, the case of DeRay Mckesson v. John Doe could prove to be a signal moment in our march to a less open, less free society.

WGBH News contributor Dan Kennedy’s blog, Media Nation, is online at dankennedy.net.