Supreme Court Justice Anthony Kennedy couldn’t hide his distaste for public-sector unions during oral arguments on Monday in the most important organized-labor case in years.

David Franklin, a lawyer representing Illinois, argued that it’s in his state’s “interest” to work with a public-sector union “that’s well-resourced enough that it can be a partner with us in the process of not only contract negotiation—”

Kennedy interrupted. “It can be a partner with you in advocating for a greater size workforce, against privatization, against merit promotion, for teacher tenure, for higher wages, for massive government, for increasing bonded indebtedness, for increasing taxes?” the justice asked. “That’s the interest the state has?”

The justice’s hostility underscored the dire straits that public-sector unions face in Janus v. AFSCME, Local 31. In oral arguments on Monday, the court’s conservative majority indicated that they were ready to strike down mandatory union fees for public employees, on free speech grounds. A final decision in the case is expected by June.

Public-sector unions rely on dues from their workers to fund their operations. In exchange, members get legal representation in the workplace, most importantly through contract negotiations with the government. About five million public employees nationwide are members of unions. In addition, Illinois and 22 other states allow public unions to charge “agency fees,” which are also sometimes known as “fair-share fees,” to non-members.

The logic behind these fees is thus: Since all public employees benefit from the union’s collective-bargaining operations, non-members shouldn’t be able to free-ride on those benefits by not paying dues. In the 1977 case Abood v. Detroit Board of Education, the Supreme Court ruled that public unions could collect agency fees from non-union employees, but unions could only use those fees for non-political functions like “collective bargaining, contract administration, and grievance adjustment.”

For Mark Janus, the Illinois public employee who filed the lawsuit, the distinction between a union’s political functions and its non-political functions is meaningless. Collective-bargaining negotiations can and do influence public policy, he argued, meaning that agency fees violate the First Amendment by compelling him to support political views he opposes. “Abood was wrongly decided because bargaining with the government is political speech indistinguishable from lobbying the government,” he told the court in his brief.

The free-speech argument resonated with the court’s conservatives. “What we’re talking about here is compelled justification and compelled subsidization of a private party, a private party that expresses political views constantly,” Kennedy told the lawyers during oral arguments. Justice Samuel Alito implicitly compared Janus’s plight to that of Thomas More, the Tudor-era English statesman executed by Henry VIII for refusing to declare the king’s supremacy over the Pope. “When you compel somebody to speak, don’t you infringe that person’s dignity and conscience in a way that you do not when you restrict what the person says?” he asked Franklin, the lawyer for Illinois. Franklin replied that compelling someone to pay a fee wasn’t quite the same as compelling someone to speak, as the court previously recognized in Abood.

Janus isn’t the first time this issue has come before the court. In the 2014 labor case Harris v. Quinn, Alito wrote a majority opinion joined by the other four conservative justices that fell short of striking down Abood. At the same time, his opinion expressed serious doubts about the precedent’s viability and called it “questionable on several grounds”—a signal flare that the court’s conservative majority was open to overturning the 1977 precedent.

Such an opportunity presented itself during the 2015-2016 term in the form of Friedrichs v. California Teachers Association, which challenged the constitutionality of agency fees for public school teachers in the Golden State. The justices appeared poised to strike down Abood until the death of Antonin Scalia in February 2016 threw the term into upheaval. The following month, the court announced it had deadlocked in a 4-4 split in Friedrichs. Under the Supreme Court’s rules, an evenly divided ruling leaves the lower courts’ decisions intact and sets no new precedents.

The split effectively punted the agency fee dispute until after the 2016 presidential election. Justice Neil Gorsuch, who was nominated by Trump and confirmed last year, was uncharacteristically silent during Monday’s oral arguments, although he’s widely expected to join his fellow conservatives in the final decision. But the court’s four liberal justices spent Monday’s oral arguments trying to highlight the potential damage of a ruling against the unions, suggesting they were on the back foot.

“What would the difficulties be, if any, if the Court were to overrule Abood?” Justice Elena Kagan asked the union’s lawyer. “What we know is that tangibly, when these kinds of obligations of financial support become voluntary, union membership goes down, union density rates go down, union resources go down,” Franklin replied. “We’ve seen it again and again.”

The court’s conservatives seemed unswayed. “Well, the argument on the other side, of course, is that the need to attract voluntary payments will make the unions more efficient, more effective, more attractive to a broader group of their employees,” Roberts asked. “What’s wrong with that?”

“The studies that I’ve read indicate that, yes, there can be an initial first flush of mobilization and organizing when something like this gets taken away,” Franklin told the chief justice, “but that over the long term, human nature and basic economics dictate that the free-rider problem will become endemic.” He added that this problem would only get worse over time as remaining members are left to carry a higher share of the costs, thus increasing pressure for them to also drop out.

It’s not hard to see the political ramifications if unions lose in Janus. With fewer resources to spend on their core functions, weakening public-sector unions could also hinder the electoral prospects of the Democratic Party, which frequently relies on organized labor’s networks and resources in its political campaigns. That’s one reason why conservative donors and think tanks helped bankroll the Janus case as it worked its way through the lower courts.

For all their protestations of political neutrality, the justices seemed well aware of the case’s potential impact. “I’m asking you whether or not in your view, if you do not prevail in this case, the unions will have less political influence—yes or no?” Kennedy quizzed Frederick, the lawyer representing the public union, at one point.

“Yes, they will have less political influence,” he replied.

“Isn’t that the end of this case?” Kennedy asked.