In the wake of a 2018 Supreme Court ruling that public-sector unions cannot force non-members to pay a fee for workplace representation, a new challenge to union power is taking shape. The Supreme Court ruled in the case of Janus v. AFSCME that unions could not extract what’s called an “agency fee” from non-members who happened to work in the same place a union had “exclusive representation” rights. But one group argues that in saying it was unconstitutional to force people to fund labor unions’ speech with their own money, the high court also indicated that the legitimacy of “exclusive...