The big Supreme Court ruling that everybody is watching for on Monday is Hobby Lobby, the lawsuit challenging Obamacare’s contraception mandate. But there’s one other, very important decision expected on the Court’s last day of the session. The case is Harris v. Quinn and it could have a damaging and long-lasting effect on public sector unions—and, potentially, the labor movement as a whole.

The plaintiff in the case is Pamela Harris, of Illinois, who provides in-home care for her 25-year-old son with disabilities and receives compensation for it from the state’s Medicaid program. This is a common arrangement, designed to make it possible to assist family members who care for relatives with serious illnesses. But in Illinois, as in a few other states, there’s a twist.

In 2003, the governor declared all home health care workers to be public employees. The theory behind the decision was that the workers are getting much of their money from the government, in the form of Medicaid. The widely understood idea was to make it easier for them to organize into a union, since government doesn’t fight organizing drives the way private companies do. The state’s employees eventually voted to do just that, joining the Service Employees International Union. As a result, Harris must pay a portion of SEIU’s membership dues—an administrative fee, in order to support the bargaining the SEIU does on behalf of employees—even if she opts not to join the organization formally. She’s not happy about that, so she filed a lawsuit. Forcing her to pay union dues, she says, violates her constitutional rights.

The question of whether it’s okay to force people to pay union dues is a long-standing controversy. Defenders of unionism, not least among them the unions themselves, argue that it’s necessary to prevent “free-riding.” When a union negotiates on behalf of its members, all workers benefit from the higher wages, better job protections, and so on. As the theory goes, all workers should have to share in the administrative costs of this work. (There’s a separate controversy over whether it’s fair to require all workers to support union political activity.)

Opponents of unionism, like the National “Right to Work” Committee (NRTWC) Legal Defense Foundation, believe such arrangements amount to coercion. The organization and its allies have persuaded 24 states to create laws allowing workers to opt out of private sector union membership and dues, and those laws are widely thought to be a big reason unions have been so weak across the South. The organization has also targeted public unions and has provided the legal counsel for Harris’ suit.