The American Civil Liberties Union told The Washington Post that it stands ready to defend, in principle, the rights of players to express themselves and to challenge Trump’s “unconstitutional efforts to bully the NFL into complying with his view of what is politically correct.”

If the NFL acts because of Trump’s threat to punish the league, players could legitimately claim that their First Amendment rights have been violated, said David Cole, the ACLU’s national legal director.

“The courts have recognized that when government officials threaten punishment or consequences because of protected speech, that in and of itself can chill the speech, in violation of the First Amendment,” Cole said, citing a 1986 case in which a federal court sided with a challenge by Playboy Enterprises against Edwin Meese, then the U.S. attorney general, for sending letters threatening to publish a list of ­7-Eleven convenience stores that sold pornography.