The immense political influence of public sector unions appeared imperiled Monday, as several justices of the U.S. Supreme Court grilled labor lawyers over the constitutionality of mandatory union dues.

Though their liberal colleagues warned of dire consequences, Chief Justice John Roberts and Justices Anthony Kennedy and Samuel Alito excoriated advocates defending forced fees.

Alito invoked the legacy of St. Thomas More, who chose to die rather than lend his name to a position he disagreed with, while Kennedy said the unions were asking the Court to ignore reality.

Also WATCH: Rebecca Friedrichs explains why she fought her union all the way to the Supreme Court

Over five million government employees are required by law to pay dues to public-sector unions. A 1977 Supreme Court precedent called Abood v. Detroit Board of Education allows labor bosses are to collect compulsory dues, known as fair share fees, from public employees since union negotiators represent all government workers for purposes of collective bargaining — even if they are not union members. The decision prohibits unions from using mandatory fees for political purposes like electioneering.

Critics of the arrangement say fair share fees violate the First Amendment, since dissenting workers are forced to subsidize speech with which they disagree. They also say the distinction Abood creates is unworkable, since all public union activity is necessarily political.

Monday’s case arose from Illinois where a state child support specialist named Mark Janus sued the American Federation of State County and Municipal Employees (AFSCME) in hopes of ending the fair share regime. Janus pays out approximately $600 per year in union fees. AFSCME is the largest public employee union in the country.

“The ruling we are seeking would not prevent a single worker from joining or paying dues to a union if they voluntarily choose to,” said William Messenger, the lawyer who represented Janus at the high court. “It would simply recognize that the First Amendment protects government employees from being compelled to subsidize union speech.”

David Frederick, who represents the AFSCME, says mandatory fees are essential for a stable workforce, and warned of widespread turmoil if the justices end the practice.

“Union security is the tradeoff for no strikes,” Frederick said as he concluded his argument. “And so if you were to overrule Abood, you can raise an untold specter of labor unrest throughout the country.”

Roberts was skeptical of Frederick’s fatalism, and suggested the labor movement would be reinvigorated in a post-Abood context.

“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,” he said. “What’s wrong with that?”

Proponents of Abood say state and local governments have a strong interest in ensuring labor peace and orderly negotiation processes. Many local officials have determined exclusive bargaining with unions is the best way to attain those goals, so courts, they say, should defer to that reasoned judgement.

Kennedy seemed outright hostile to this line of argument, suggesting it was a poorly-disguised ex post facto explanation for political coalition building. He said many municipal governments align with union locals simply because they are useful partisan allies.

“Doesn’t it blink reality to deny that that is what’s happening here?” he asked.

Alito, a longtime critic of Abood, told Frederick he was amazed a union lawyer would argue against extensive free speech protections for workers.

“When I read your brief I saw something I thought I would never see in a brief filed by a public employee union, and that is the argument that the original meaning of the Constitution is that public employees have no free speech rights,” he said.

Justice Elena Kagan pointed out that hundreds of state and local governments operate on the assumption that fair-share fees are lawful. A ruling to the contrary could be deeply disruptive to pre-existing contracts and workplace procedures.

“So the property and contract rights, the statutes of many states and the livelihoods of millions of individuals [would be] affected all at once,” she said. The point seemed like an appeal to her most conservative colleagues, who care deeply about property and contract protections.

A majority of the Court evinced discomfort with Abood in a pair of labor cases argued in 2012 and 2014. The justices heard a direct First Amendment challenge to fair share fees in January 2016, but Justice Antonin Scalia’s death shortly thereafter precluded the court from definitively resolving the controversy. Without Scalia, the tribunal split four to four so a binding decision was never issued.

The Court was restored to its full complement of nine in April 2017 with Justice Neil Gorsuch’s confirmation, and the bench agreed to revisit the issue. If the justices again divide along ideological contours, Gorsuch will give the anti-Abood conservatives the fifth vote they need to strike down mandatory dues.

Gorsuch did not offer clues as to his thinking about the case. He was silent during Monday’s arguments and has not written extensively on this subject during his career on the federal bench.

A decision in the case, Janus v. AFSCME, is expected by June.

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