In Monday’s oral arguments at the US supreme court, US solicitor general Donald B Verrilli Jr sought to put the court’s five conservative members on the spot. Verrilli did this in a closely watched case in which 10 California teachers assert that being forced to pay union fees violates their first amendment rights.

Backing the teachers’ union in the case, Verrilli pressed the court’s conservative members to justify their apparent move toward overturning a unanimous 1977 supreme court decision, Abood v Detroit Board of Education, that upheld a requirement that public school teachers pay union fees, even when they opt out of joining the union. Underlining the gravity of overturning a decades-old ruling, Verrilli said: “We’re talking about overruling a precedent of 40 years’ standing. There needs to be a showing of changed circumstances.”

Labor unions grew alarmed last June when the court agreed to hear the case, Friedrichs v California Teachers Association, fearing that the justices would hobble public-sector unions by barring any requirement that government employees pay fees to the unions that represent them. In states that give public-employee unions a right to bargain, but prohibit these fees (known as fair share fees or agency fees), 34% of teachers opt out of paying such fees. Since public-sector unions are one of the Democratic party’s most generous backers, many Democrats fear their party will be weakened if the justices bar fair-share fees.

Verrilli may well have been aiming his comments on the need to show “changed circumstances” to Justice Antonin Scalia. Though a leader of the court’s conservative wing, Scalia was considered a potential swing vote in Friedrichs, who might back the union’s position. In 1991, Scalia delivered a robust defense of fair-share fees, writing: “Where the state imposes upon the union a duty to deliver services, it may permit the union to demand reimbursement for them … where the state creates in the nonmembers a legal entitlement from the union, it may compel them to pay the cost.”

In Abood, Justice Potter Stewart, an Eisenhower appointee, wrote of “the great responsibilities” that unions have representing workers, duties that “often entail expenditure of much time and money”, including the “services of lawyers, expert negotiators, economists, and a research staff”. Stewart concluded that it was fair and not a first amendment violation to require government employees to pay fees to the unions that represent them. In Abood, the court also ruled that government employees can’t be required to pay union fees that are spent on political matters, as opposed to collective bargaining.

On Monday, Scalia seemed to have significantly changed his tune since 1991, although he didn’t explain why or point to any changed circumstances. From the start that day, he showed hostility to fair-share fees, asserting that every issue that public-sector unions bargain about is essentially political – and workers shouldn’t be forced to pay union fees over “political” matters.

“Everything that is collectively bargained is within the political sphere, almost by definition,” Scalia said. “Should the government pay higher wages or lesser wages? Should it promote teachers on the basis of seniority?”

In one changed circumstance since the 1977 Abood ruling, the court’s conservative majority showed little concern about the “great responsibilities” and significant expenditures unions face in bargaining for workers, even those who opt out of joining.

In recent years, Justice Samuel Alito has led a not-so-quiet campaign to chip away at Abood and get the court to rule that requiring public employees to pay union fees violates first amendment free speech rights. On Monday, not just Scalia, but Chief Justice John Roberts and Justice Anthony Kennedy lent support to Alito’s position. (Justice Clarence Thomas remained silent.)

Scalia, echoed by Roberts, said government employee unions could survive and be effective even if workers were no longer required to pay agency fees. Indeed, Roberts argued that if unions do a good job, they should be able to convince the great majority of workers to continue paying union fees.

But Edward Dumont, California’s solicitor general, warned the justices of a “free rider” problem. Even if workers think having a union is “very advantageous”, Dumont said, “if they are given a choice, they would prefer to have it for free, rather than to pay for it.”

Verrilli said that before fair-share requirements were established, union officials often demonized management and whipped up workers against their employer, to persuade more workers to join and pay union fees. David Frederick, the lawyer for the California Teachers Association, argued that requiring all workers to pay union fees fosters labor peace and stability “by making a shared sacrifice for the purposes of working together to establish a coherent position with their employer”.

But Scalia scorned that view, saying: “You say that, but it doesn’t mean anything to me.”

It was hard to read Scalia’s mind as to why he has apparently abandoned his position in favor of fair-share fees. Is it part of the Republicans’ growing disaffection with labor unions over the past quarter century? Has Scalia been pulled along by Alito in his crusade against agency fees? Or does Scalia want to help ensure that the conservative majority delivers another powerful ruling to advance Republican political interests, much like Bush v Gore, Citizens United as well as the Voting Rights Act and voter ID cases?

Considering Scalia’s newfound hostility toward fair-share fees, many court observers were convinced a majority of the court would rule them unconstitutional – in essence creating a nationwide “right to work” law for government employees. Perhaps making things worse for labor, Justice Kennedy signaled on Monday that he thought it might also be a first amendment violation for states to require private-sector workers to pay agency fees. That could open the door to a follow-up lawsuit that could be devastating to labor – one that sought to bar agency fees in the 25 non-right to work states that allow such fees in private-sector unionized workplaces.