UNIONS have never been uncontroversial in American society, but the battles over labor have grown fiercer in recent years: Witness the fight over public-employee unions in Wisconsin, or the 2012 decision by Michigan lawmakers to join the ranks of “right to work” states.

On Monday a 5-to-4 majority of the Supreme Court fired its own salvo in the war on unions. Though its decision in Harris v. Quinn was narrow, saying that, in some cases, unions could not collect fees from one particular class of public employees who did not want to join, its language suggests that this may be the court’s first step toward nationalizing the “right to work” gospel by embedding it in constitutional law.

The petitioners in Harris were several home-care workers who did not want to join a union, though a majority of their co-workers had voted in favor of joining one. Under Illinois law, they were still required to contribute their “fair share” to the costs of representation — a provision, known as an “agency fee,” that is prohibited in “right to work” states.

The ability of unions to collect an agency fee reflects a constitutional balance that has governed American labor for some 40 years: Workers can’t be forced to join a union or contribute to its political and ideological activities, but they can be required to pay for the cost of the union’s collective bargaining and contract-administration activities.