But in 22 other states, including California, New York and Pennsylvania as well as Illinois, the law allows employees to form a union which in turn has a legal duty to represent all the employees.
In those states, school boards, transit districts, police departments and state agencies may negotiate contracts that require all workers -- even those who do not join the union -- to pay a so-called "fair share fee" for the benefits they would receive along with union members, such as higher pay scales.
More than 40 years ago, the Supreme Court gave this arrangement its constitutional blessing. The justices set out a middle position in the case of the case of Abood v. Detroit Board of Education. They said public employees have a free speech right to opt out of paying the full dues to a union if some of the money is spent for political contributions or lobbying. However, the court said, they may be required to pay a lesser fee to support the union's workplace activities. Otherwise, "free riders" could benefit from a better contract, but pay nothing.
The Illinois lawsuit asks the court to overturn the Abood decision and strike down forced union fees nationwide.
Soon after taking office in 2015, Rauner had tried to block union fees through an executive order, and when that failed, he filed a suit in federal court contending the payments were unconstitutional.
Illinois' Democratic Attorney General Lisa Madigan intervened to defend the state's labor law, and a judge ruled the governor had no standing since he was not paying the fees. But the suit continued after his lawyers substituted as a plaintiff Mark Janus, a child support specialist. He works for a state agency in Springfield and objects to the $45 fee he pays each month to the American Federation of State, County and Municipal Employees.
The union "takes political positions that he doesn't support. They advocate for more spending and higher taxes," said Jacob Huebert, a lawyer for the Liberty Justice Center who represents Janus.
For its part, AFSCME called the case "a political attack on the freedoms of working people by the same corporate billionaires and corporate interests that have for years rigged our economy and politics in their own favor."
Rauner's challenge to union fees is likely to win favor from the court's five more conservative justices, all of them Republican appointees. Two years ago, the court was set to strike down mandatory union fees in a case brought by a California schoolteacher. But the sudden death of Justice Antonin Scalia left the court split 4 to 4.
Once Justice Neil M. Gorsuch, appointed by President Trump, was confirmed to fill Scalia's seat, the court said it would decide the union fees issue in the case from Illinois.