Legally, this should be an easy win for the unions. The Supreme Court upheld fair-share fees four decades ago in a unanimous ruling it has reaffirmed repeatedly, and on which more than 20 states have relied in negotiating thousands of contracts covering millions of public employees, including firefighters, teachers and police officers. The logic is simple: When the government is an employer, it has more control over its employees’ speech than over that of regular citizens. Any burden the fees impose on employees’ First Amendment rights is justified by the need to eliminate free riders — workers who enjoy union benefits without having to pay for them, which can deplete the unions’ resources in states where they are legally required to represent all workers, members and nonmembers alike. Anti-union advocates dismiss the free-rider concern, but it’s very real: In states that have ended the fees, more than one-third of public-school teachers are free riders.

None of this seems to register with Justice Samuel Alito Jr., who has made no secret of his dislike for that 1977 opinion, Abood v. Detroit Board of Education, and has been searching for the votes to overturn it for at least six years, writing opinions intended to set up its demise. Justice Alito probably assumed he had victory in hand in 2016, when the court considered the same question in a case brought by California public-school teachers against their union. But when Justice Scalia, whose remarks during oral arguments strongly suggested he would provide the fifth vote against the union, died a few weeks later, the case deadlocked.

As it turned out, Justice Scalia’s death only briefly slowed the march of corporate interests that have sought for years to protect their huge profits and kill off the last remnants of organized labor in America. They and the right-wing groups that support them quickly regrouped around another plaintiff, a child-support specialist from Illinois named Mark Janus, who makes the same argument the California teachers did: Activities by public-sector unions are inherently political, so it’s impossible to separate the costs of collective bargaining from those of political lobbying. No matter what benefits the unions negotiate on his behalf, he shouldn’t have to pay a dime. Mr. Janus lost decisively in the lower federal courts for the same reason the teachers did: the Supreme Court’s precedent in the Abood case.

That precedent should guide the justices here, too. Beyond the court’s core principle of stare decisis, which keeps it from overturning its own prior decisions except in extraordinary circumstances, the Abood decision reasonably balanced workers’ First Amendment rights against the government’s interest in labor peace. Two notable conservative scholars go further, arguing in a supporting brief to the court that the First Amendment does not apply at all in this case. The government is allowed to compel subsidies of others’ speech all the time without violating the Constitution, they point out, such as by collecting and spending taxes.