The presidents of America’s two largest unions have vowed to fight tooth and nail against a lawsuit now in front of the U.S. Supreme Court which, if successful, could strike down a California law permitting mandatory union membership. The plaintiffs in the lawsuit assert that the mandatory-dues law forces teachers to finance union political positions. Thus, the plaintiffs say, the law violates the First Amendment rights of teachers who disagree with the union’s politics. The Supreme Court granted certiorari for the suit, Friedrichs v. California Teachers Association, on Tuesday. “We are disappointed that at a time when big corporations and...