Kermit Roosevelt, an expert on retroactive constitutional claims at the University of Pennsylvania Law School, said Mr. Mitchell’s legal reasoning “would be very destabilizing” beyond union matters if it gained significant traction in the court system.

For instance, he said, if a future Supreme Court struck down affirmative action in college admissions, applicants denied admission under the previous policy would be able to win damage awards retroactively tied their rejection.

Courts have been sympathetic to same-sex spouses seeking to recover benefits or property they were denied before the Supreme Court’s recent decisions on same-sex marriage. But courts have specifically rejected such retroactive arguments — so far — in the context of union fees. After a 2014 ruling in which the Supreme Court struck down mandatory union fees for home-based workers paid through government programs like Medicaid, federal district judges dismissed at least three lawsuits seeking refunds of workers’ mandatory fees. (A fourth case that had begun earlier was settled after a judge denied the plaintiffs’ class-action certification.)

In all three cases, the judges found that the unions could claim a good-faith defense because they had followed “seemingly valid state laws” requiring the fees, as one judge put it, that were only later deemed to be unconstitutional. No appeals court has overturned any of these rulings.

Even so, Mr. Mitchell and his allies may get a favorable reception in the one court that really matters: the Supreme Court.

“This court has shown itself to be so hostile to workers’ rights that they will find a way,” said Sharon Block of the Labor and Worklife Program at Harvard Law School, who is a former senior Labor Department official and National Labor Relations Board member.

Image Jonathan F. Mitchell, a conservative lawyer, is the lead counsel in several lawsuits against public-employee unions. Credit... Courtesy of Jonathan F. Mitchell

Will Baude, a libertarian-minded professor of constitutional law at the University of Chicago, said the Supreme Court had proved less sympathetic than some lower courts to good-faith defenses for private parties like unions.