MARK JANUS could be making history this year. On February 26th the social worker from Illinois will be sitting with his two lawyers in the hallowed setting of the Supreme Court as the justices hear one hour of oral arguments in Janus v American Federation of State, County and Municipal Employees, which asks whether public employees like himself, who choose not to join their designated union, may still be charged a compulsory “agency fee” to support collective bargaining. Mr Janus argues that the fee violates his First Amendment right to freedom of speech, because it forces him to subsidise an organisation whose bargaining position he rejects.

The court’s ruling in the case could determine the future of the labour movement. Though unionism has been in decline for decades, the public sector has remained a stronghold of organised labour. Eighty-four percent of workers today are employed in the private sector; only around 6% are union members, compared with about one-third in the 1960s. Yet of the 16% of employees in the public sector more than one-third are still unionised. For some professionals, such as teachers and firefighters, the share is higher.

The relative strength of public-sector unions and their inveterate support of Democratic policymakers have made them a prime target of Republican governors, especially in the Midwest. Scott Walker, the governor of Wisconsin, risked a recall election in 2012 (which he won) when he brought in Act 10, a law which all but eliminated collective-bargaining rights for 175,000 public-sector employees. Mr Walker went for public-sector unions before he took aim at those in the private sector. In 2015 Wisconsin adopted a “right-to-work” (RTW) law, which stipulates that private-sector workers cannot be forced to join a union, and pay dues, as a condition of employment. The same year, in next-door Illinois, Governor Bruce Rauner, another Republican, asked a federal court to declare agency fees unconstitutional. Robert Gettleman, a judge in Chicago, dismissed Mr Rauner’s complaint. But he allowed Mr Janus’s suit to proceed. It was pushed by the National Right To Work Legal Defence Foundation (NRTW) and the Liberty Justice Centre, two non-profit groups, after they realised that Mr Rauner was not getting anywhere with his suit.

If the court rules in favour of Mr Janus, public-sector unions everywhere in America will no longer be authorised to collect agency fees. Twenty-eight states, including Indiana, Iowa, Kentucky and Michigan, have adopted RTW laws—and some states have done away with collective-bargaining rights for certain public-sectors workers such as teachers in Tennessee, municipal employees in Oklahoma and farmworkers in Maine. But in 22 states unions still collect compulsory fees from around 5m public-sector workers. The big question is how many of them would continue to pay if the fee became optional.

Paul Secunda at Marquette University Law School in Wisconsin is convinced that a pro-Janus ruling would have a devastating effect on public-sector unions, as it would deprive them of a big chunk of their income. Thanks to Governor Walker pushing through Act 10 and the RTW law, membership of both public and private-sector unions has plummeted in Wisconsin. According to the Bureau of Labour Statistics, union membership in the state has declined from 14% in 2010 to just 8%.

Their supporters say unions perform essential services for their members, who would be far worse-off without them. According to the Economic Policy Institute, a think-tank, workers covered by union contracts earn on average 13% more than their peers in a workplace that is not unionised. Ninety-four per cent receive access to employer-sponsored health benefits, compared with just 67% of non-union workers, with union employers paying 77% more (per hour worked) towards their employees’ health coverage than comparable non-union employers.

Unionised workers get more paid sick days and paid holidays. They are half as likely as their non-unionised peers to be paid less than a state’s minimum wage, a situation that can arise in jobs where workers depend on tips. And if the share of unionised workers is high, wages of non-union workers can increase too. In 2014 the Services Trade Council Union (representing housekeepers, lifeguards and other service workers) and Disney World agreed on wage increases for union members at its Orlando theme park to at least $10 an hour, starting in 2016. Disney subsequently extended the wage increase to all its 70,000 employees in Orlando.

Both sides of the argument expect the Supreme Court to rule in favour of Mr Janus in June. When a very similar case was argued at the court in 2016, the justices tied 4-4 because of the death of Antonin Scalia, the court’s chief conservative voice. His replacement, Neil Gorsuch, is expected to side with Judge Samuel Alito and the other three conservative justices. Mr Janus probably never expected to play such an important role in the history of America’s labour movement.