Some blockbuster Supreme Court decisions are easy to see coming, while others emerge unexpectedly. It is easy to foresee that last week’s case on the president’s power to make recess appointments will yield an opinion of far-reaching significance. This week, however, the Supreme Court heard arguments in a case that at first seemed slight, but which may turn out to be a blockbuster.

At issue in the case of Harris v. Quinn is whether home health care aides in Illinois, who do not want to be represented by the Service Employees International Union (SEIU), can be made to pay “fair share” fees to the union.

Under Illinois law, even workers who don’t want to be union members get the same pay, working conditions and benefits won by the union negotiators for their members. Because non-members pay no union dues, the law requires them to pay the disputed fees, which are supposed to cover only the “fair share” of the costs incurred by the union in negotiating on behalf of non-members, but not to support any political activities in which the union also might engage.

The case at first seemed to be relatively minor. The main question seemed to be whether these home health care workers really are state employees, who would have to be represented by the SEIU, or whether they are private contractors, who would not. On the one hand, the home health aides are hired by the caregivers and their families and so seem like private employees, but they are paid by the state through Medicaid, and so in a way are public employees.

The oral argument on Tuesday, however, focused not on this narrow question about how to classify these health care workers but on the larger question of whether public employee unions generally have the right to charge these “fair share” fees. That right was affirmed by the Supreme Court in a 1977 case, but Tuesday’s argument suggested that the court is now thinking about overruling that decision.

Defenders of the public employee unions argue that public unions should be treated just the same as private unions, and that, when they act as employers, state governments should be treated as much as possible like private employers. In the private sector, when a union is named the exclusive representative of all the employees — both members and non-members — the union is allowed to charge non-members these “fair share” fees to protect against “free riders.”

Where unions are both required to represent everyone but are denied the right to collect such fees, every individual union member has a financial incentive to quit the union. If you can get the same benefits and pay as everyone else, without paying union dues or other fees — if you can get a free ride — why pay?

If enough people quit, however, the union would go broke. So, the logic goes, if there are to be union shops, there also must be these supplementary “fair share” fees. Otherwise, the continued existence of public employee unions might be threatened.

Opponents argue that public employers are different. When a teachers’ union, for example, seeks or opposes merit pay, or when it seeks or opposes changes to tenure rules — things it would have to do in negotiating employment contracts — it also would necessarily be taking a stand on politically weighty questions on which many Americans are divided.

Whether our government should be larger or smaller is among the biggest questions now dividing Democrats from Republicans. Opponents of the “fair share” fees contend that public employee unions cannot avoid taking positions on these questions in conducting labor negotiations. Thus, as they see it, the “fair share” fees necessarily compel public employees who do not share the union’s political outlook to pay for political activities they do not support.

Normally, the First Amendment protects us from being compelled to support political causes, and the right of each individual to express his or her own political opinions is fundamental to our constitutional system.

On Tuesday, three justices — Anthony Kennedy, Samuel Alito and Chief Justice John Roberts — seemed worried that the “fair share” fees might violate the First Amendment. Justices Stephen Breyer, Ruth Bader Ginsburg, Elena Kagan and Sonia Sotomayor sounded more likely to reaffirm their legitimacy. Assuming that Justice Clarence Thomas will side with the other conservatives, that leaves the fate of the “fair share” fees, and perhaps of public employee unionism, in the hands of Justice Antonin Scalia, who asked many questions on Tuesday but, unusually for him, gave no hint of his eventual decision.

Joseph R. Reisert is associate professor of American constitutional law and chairman of the department of government at Colby College in Waterville.