SOMEWHERE among the thousands of petitions the justices will wade through when they return to work in September is a case that could transform the labour movement in America: Janus v American Federation of State, County and Municipal Employees (AFSCME). The Supreme Court refuses to hear about 99.4% of appeals at this time of year—a rejection rate even stingier than its usual 89.9%. But Janus is very likely to attract the four votes it needs to make it onto the court’s docket for the upcoming term that begins October 2nd.

The dispute revisits a question the Supreme Court answered 40 years ago in Abood v City of Detroit Board of Education: whether public-sector unions may charge a fee to non-members for the cost of negotiating their contracts. The unanimous court in Abood began with the premise that many states require all workers in a particular sector to be represented by a union and that members and non-members alike benefit from their work. Given this arrangement, the court reasoned, so-called “agency” or “fair-share” fees preserve “labour peace” and prevent employees from hitching a free ride on the backs of their dues-paying colleagues. Teachers, firefighters, policemen and other public employees are not required to join a union, and they cannot be forced to contribute to a union’s political or ideological work, Abood held. But they can be charged a fee for the union’s efforts to bargain for their salary and benefits.

Mark Janus, a child protective services employee in Illinois, is represented by and pays agency fees to AFSCME. But Mr Janus, who thinks AFSCME has contributed to his state’s “budget and pension crisis” by backing spendthrift candidates and pushing for fiscally irresponsible contracts, says “the union’s voice is not my voice” and “the union’s fight is not my fight”. As his petition to the justices reads, the Abood rule requires him “to subsidise AFSCME’s efforts to compel the state of Illinois to bend to the union’s will” regarding a series of proposed cost-saving reforms. According to his lawyers, that violates his First Amendment freedom of expression.

This may sound familiar. Two years ago, in Friedrichs v California Teachers Association, the Supreme Court took up the complaint of Rebecca Friedrichs, a disgruntled public-school teacher in California who sued her union for compelling her to pay agency fees. The oral argument in January 2016 seemed to herald a win for Ms Friedrichs. Justice Samuel Alito—who wrote an opinion in 2012 calling Abood “something of an anomaly” in First Amendment law, all but inviting lawsuits to challenge it—was clearly in Ms Friedrich’s camp. The other conservative justices seemed to support her position, too, as did the frequent swing voter, Justice Anthony Kennedy. He said it made “no sense” to tell teachers who “strongly, strongly disagree with the union position on teacher tenure, on merit pay, on merit promotion, on classroom size” that they need to pay the union to advance such positions.

Before the Supreme Court could issue a ruling in Friedrichs, the conservative Justice Antonin Scalia died, leaving the court split 4-to-4. That tie affirmed the lower court’s ruling in favour of the union, leaving agency fees intact for the time being. But Mr Janus’s case was already working its way through the federal courts when Friedrichs ended in impasse. Now those who would like to see the end of agency fees for the 7.8m public employees in 22 states where unions are authorised to collect them will probably have another shot. Barring a late-breaking change of heart by one of the conservative justices, Mr Janus seems very likely to score a victory should the Supreme Court agree to hear his case.

Everything seems to turn on Donald Trump’s choice to fill Justice Scalia’s seat: Neil Gorsuch. In the two-and-a-half months he sat with his eight colleagues in the term that ended in June, Justice Gorsuch established himself as perhaps the most conservative member on the high-court bench. But is he bold enough to overturn a long-standing court precedent so early in his tenure? In his Senate confirmation hearings, Mr Gorsuch professed an allegiance to stare decisis, the idea that justices should typically abide by the court’s prior rulings. He said that a ruling’s age, the degree to which the country relies on it and the solidity of its legal foundation all had to be weighed before jettisoning it.

Four decades is a long time, and Abood’s agency-fee rule has defined the way public-sector unions do business in about half of American states, including populous ones like New York and California. A win for Mr Janus could reduce union membership as employees seek to reap the benefits of representation without paying out of pocket. This fear is spurring some unions to pre-emptively shore up support among their rank-and-file; AFSCME, for one, is asking members to pledge to stick by the union through thick or thin. Yvonne Walker, president of SEIU Local 1000, California's largest public sector union, is worried about the broader effects of an unfavourable ruling. She says workers in Wisconsin, which abolished mandatory dues in 2015, have seen cuts to their "wages, health insurance, and...retirement" leading to "higher poverty rates and increases in work place deaths".

If the Supreme Court ultimately nixes mandatory union fees, the unions’ appeals to their members may soften the blow somewhat. But the plaintiffs’ goal, according to Illinois governor Bruce Rauner, goes far beyond winning more control over state budgets. He says that if Mr Janus wins his case, this “will change the culture and the power structure...in Illinois” and “across America”.