President Donald Trump can’t carry out his threat to take away the National Football League’s tax-exempt status aimed at making the league force players to stand for the national anthem. That would be a clear violation of the First Amendment.

It’s true that the NFL had voluntarily given up its tax break two years ago, which means that Trump’s threat wouldn’t have practical effect even if he could make it stick. But as the Supreme Court recently held, even an effort to violate free speech based on a factual mistake is illegal.

And the president’s action still matters because of the chilling effect it could have on speech of other entities that really are vulnerable to presidential action.

Behind this episode lies an important constitutional question:

Does the First Amendment govern presidents who use the bully pulpit to act like bullies? Or is it OK for presidents to use their own free speech to encourage others to follow a particular course of action?

The answer is that the First Amendment bars presidential bullying that includes a concrete threat to take government action against a private citizen or group in order to coerce speech.

Start with Trump’s threat. On Tuesday he tweeted:

“Why is the NFL getting massive tax breaks while at the same time disrespecting our Anthem, Flag and Country? Change tax law!”

The basis for the threat was the fact that, since 1942, the NFL’s league office — not the teams — has been treated as a trade association by the Internal Revenue Service, conferring a tax exemption under section 501(c)(6) of the tax code.

The IRS answers to the president. This would seem to give the White House the power to order the IRS to reconsider or alter the league’s tax status. Apparently Trump didn’t know that the football league, concerned about public criticism, had voluntarily decided to forgo tax-exempt status in 2015.

Still, the president’s statutory power means his threat wasn’t obviously empty.

(By contrast, Trump has less power to carry out his threat on Wednesday to challenge NBC’s broadcast license because of news stories he disputes. That’s because broadcast licenses are controlled by the Federal Communications Commission, an independent body that doesn’t answer to the White House, though the president does appoint its members.)

Tax breaks for sports leagues remain a live issue. As of 2015, when Congress considered a bill to end the exemption for professional sports leagues, the National Hockey League, the Professional Golf Association Tour, and the Ladies Professional Golf Association all enjoyed tax-exempt status.

And regardless of whether the NFL would have been economically affected by the end of a tax break it didn’t use, Commissioner Roger Goodell and the league seem to be caving in to pressure. Goodell on Tuesday sent a letter to the owners suggesting a rule change to require players to stand.

Constitutionally speaking, it didn’t have to. In 2016, the Supreme Court held that the government violates free-speech rights even when its actions are based on a factual mistake. The case, Heffernan v. City of Paterson, New Jersey, involved a police officer who as a favor to his mother picked up a political sign promoting the mayoral candidacy of an opponent of the police chief. The chief, mistakenly believing the officer was supporting his rival, demoted the officer.

Justice Stephen Breyer wrote for the court that “the government’s reason … is what counts here.” The First Amendment bars the state from attempting to punish free speech — even when the attempt is based on a mistake like Trump’s.

The lesson is that when it comes to the First Amendment, motive matters. Trump’s motive in issuing his threat was to bully the NFL into forcing its players to stand during the anthem. That means Trump was threatening government action in order to suppress public speech that he doesn’t like.

That’s a clear abuse of power. The government can’t use its authority to pressure people to say what it wants them to say.

A president does have the have the right to announce that he thinks players should stand during the anthem. Even when he said he wished that the owners would fire players who took a knee, he was within his rights and was not violating anyone else’s.

But there’s a crucial constitutional difference between the president expressing his views and threatening to use the government’s power.

Trump’s tweet may seem like a small thing. But the First Amendment can be eroded by small actions. Incremental government threats can add up to a significant chilling effect. Right now, every trade association that needs IRS approval to maintain its tax-exempt status must be asking itself how to avoid getting on the president’s bad side.

Trump may not know what he’s talking about. But he definitely knows what he’s doing. And that’s why it’s important to call out his First Amendment violation when it comes to the NFL.

Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His seven books include “The Three Lives of James Madison: Genius, Partisan, President” and “Cool War: The Future of Global Competition.”