The Illinois man who won a landmark Supreme Court case that struck a blow against government unions is back in court fighting to recover the money seized from his paychecks, one of many lawsuits that could cost labor groups more than $100 million.

Mark Janus is asking a federal appeals court to force his former union to return the money that it unconstitutionally deducted from his paychecks. The former child support specialist was the named plaintiff in 2018's Janus v. American Federation of State, County, and Municipal Employees, Council 31 Supreme Court decision, which declared that public-sector unions may not forcefully collect union contributions without employee consent. Associate Justice Samuel Alito affirmed in the Court's majority opinion that forcing employees to pay partial dues known as "agency fees" to their unions violates the First Amendment.

Janus is still locked in a legal battle, seeking to recover the thousands of dollars that the union forcefully collected from his paychecks before the 2018 decision. A panel of judges on the Seventh Circuit Court of Appeals recently ruled that unions don't have to refund dues collected in compliance with the pre-Janus legal statutes. Janus has appealed that judgment, asking for an en banc review to be carried out from every judge on the circuit.

"The Supreme Court agrees with me—the union was wrong to take money out of my paycheck without my permission," Janus said in a statement. "The union knew what it was doing was wrong. The union shouldn't get to profit from behavior that the Court recognized as unconstitutional."

The union did not return request for comment about the case.

Janus's current court battle could have significant ramifications on the bottom lines of unions. If AFSCME Council 31 is forced to retroactively return money it took from Janus's paychecks, it would strengthen the cases of dozens of class-action lawsuits filed by workers across the country, according to Patrick Semmens, a spokesperson for the National Right to Work Foundation. The foundation is representing employees in more than a dozen lawsuits that could force labor groups to refund $120 million of past dues and fees to workers. Workers in several other states and cities across the country filed similar suits.

"There is probably another 20 or 30 cases that aren't ours … likely [worth] hundreds of millions of dollars," Semmens said.

Janus is seeking to recover about $3,000 he paid the union between March 2013 and June 2018, the month the Supreme Court declared forced dues illegal. Semmens said that the suit does not seek refunds for union dues before 2013, because it would be beyond the statute of limitation.

A lower court and the appellate panel have ruled that Janus's union acted in good faith and followed legal precedent in collecting mandatory dues before June 2018, limiting the retrospective applicability of the Supreme Court decision.

Janus's legal team believes the court erred in its judgment because legal precedents established the retroactive applicability of Supreme Court rulings. Semmens said that the Janus decision recognized that the Supreme Court put public sector unions "on notice" that the mandatory dues might be constitutionally suspect as early as 2012's Knox v. Service Employees International Union, Local 1000.

"When the Supreme Court rules, they don't change the law, they say this is what the law has always been," Semmens said. "This is what the First Amendment has always protected."

Four Trump-nominated federal judges sit on the Seventh Circuit. It has nine Republican-appointed and two Democrat-appointed members.

The suit is the latest in a string of legal disputes testing the limits of the Janus decision. Alaska Republican governor Mike Dunleavy recently cited the ruling when he signed a new policy requiring government employees to annually reaffirm their union membership before unions can deduct dues, triggering union lawsuits. Meanwhile, Pennsylvania's unions agreed to end a practice that gave workers only a short time to opt out of membership following a lawsuit from dissident employees.

Semmens said his client is prepared to return to the Supreme Court to secure his refund if that's what it takes.

"This exact case has been to the Supreme Court once," he said. "Whichever way it comes out at the court of appeals here, I suspect the side that doesn't win would ask the Supreme Court to take it."