An anti-union advocate who won a landmark Supreme Court case two years ago is now asking the court to order a public sector labor group to pay back the union dues that it had ruled unconstitutional.

Lawyers for Mark Janus, a former child support specialist in Illinois, filed a petition on Monday asking the high court to follow up on its ruling that public sector employees cannot be forced to pay union dues. Janus is appealing an appeals court ruling that held that the American Federation of State, County and Municipal Employees (AFSCME) does not have to pay him back for fees that it collected before the Supreme Court ruling.

“Mark Janus is just one of many public employees whose money was illegally taken by government unions,” Patrick Hughes, president of the Liberty Justice Center, one of the groups representing Janus, said in a statement. “Workers across the country are rightfully asking for their money back. It is time for the U.S. Supreme Court to weigh in on this issue and finally hold unions accountable for their years of unconstitutional behavior.”

AFSCME general counsel Judith Rivlin said in a statement that such cases have largely failed in the courts.

“Federal courts across the country continue to reject these attempts by corporate interests to manipulate the judiciary against working people and trample on their rights and freedom to join together in a union,” Rivlin said. “Working people have won every single case thrown at them by these special interest groups, and if judges continue to weigh these cases on the facts and merits, the corporate interests behind them will continue to fail in their efforts to further rig the system in their favor.”

AFSCME argued before the Seventh Circuit Court of Appeals that at the time those fees were collected, they were “indisputably lawful” given past Supreme Court precedent that was not overturned until 2018.

“Plaintiff’s affirmative argument that the Union is liable for fees that it received at a time when it was constitutional to do so rests on little more than the assertion that ‘Supreme Court decisions are retroactive,’” AFSCME wrote in a court filing, calling that view an “overly superficial” understanding of the court’s jurisprudence.

In 2018, the Supreme Court ruled in a 5-4 decision carried by its conservative majority that government workers who choose not to join a union cannot be forced to pay dues to the groups negotiating with their employers on their behalf. With that ruling, the majority overturned a 40-year-old precedent, finding that such arrangements violated the First Amendment.

In a dissent, Justice Elena Kagan argued that the decision could cripple government workers’ representation and upend legal frameworks around the country built on the Supreme Court’s decades-old decision.

“There is no sugarcoating today’s opinion,” Kagan wrote. “The majority overthrows a decision entrenched in this Nation’s law— and in its economic life—for over 40 years. As a result, it prevents the American people, acting through their state and local officials, from making important choices about workplace governance. And it does so by weaponizing the First Amendment, in a way that unleashes judges, now and in the future, to intervene in economic and regulatory policy.”

It’s unclear whether Janus’s latest appeal will sway four justices to agree to hear the case. With the Supreme Court nearing the end of its oral argument docket, the case would likely not be heard until the next term, which begins in October.

Updated at 5:48 p.m.