Several conservative justices had skeptical questions for lawyers supporting the union position, while the liberal justices pushed back at the notion of the Court overturning long standing precedent.
That precedent holds that public unions cannot charge nonmembers for ideological or political advocacy, but they can charge for collective bargaining that goes to such issues as wages and grievances.
Supporters of public unions fear that the Court could wipe out that almost 40-year precedent and deal a blow to labor at a time when unions in general are in a decline. They see the case as an unfair attack from conservative legal advocacy groups seeking to weaken organized labor that largely, although not exclusively, supports Democrats. Thousands of contracts affecting potentially millions of people could be in play.
The challenge is brought by a group of public school teachers in California -- backed by the libertarian Center for Individual Rights -- who are required by California law to contribute to fees germane to collective bargaining.
They argue that the compelled fees violate their First Amendment rights, and in court, they seemed to attract the support of several of the conservative justices.
Justice Antonin Scalia suggested more than once that in the public sector it is difficult to distinguish between union expenditures that are made for collective bargaining purposes and those what are made to achieve political ends.
"The problem is that everything that is collectively bargained with the government is within the political sphere almost by definition," he said ticking off issues such as wages and seniority.
Chief Justice John Roberts seemed dismissive of any notion that the nonunion members needed to pay their so called "fair share" to support collective bargaining done for their benefit.
"If your employees have shown overwhelmingly that they want collective bargaining," he said to a lawyer for the State of California, "then it seems to me the 'free-rider concern' that has been raised is really insignificant," he said.
Justice Anthony Kennedy pushed hard on the notion that the teachers bringing the challenge strongly disagree with the union position on some issues covered by collective bargaining.
"When you are dealing with a governmental agency, many critical points are matters of public concern," he said. "And is it not true that many teachers strongly disagree with the union position on teacher tenure, on merit pay, on merit promotion, on classroom size?"
Kennedy was animated at times discussing the impact of the fees on the First Amendment rights of the challengers. "You're again talking about a whole class of persons whose speech has been silenced," he said.
Justice Samuel Alito picked up on a line of questioning he has asked in previous cases questioning whether the line drawn in the 1977 case called Abood v. Detroit Board of Education is "workable."
At one point, Kennedy, did inquire about the impact a decision against the union would have on existing contracts, an issue that dominated the questions by Justice Elena Kagan, who asked about the "thousands" of contracts that might be in jeopardy. Kagan and Justice Stephen Breyer also stressed the impact of the court taking the rare step of overturning Abood.
Breyer noted that Abood was a "compromise" when it was decided but it would be a "big deal" if the court decided to overturn it.
Twenty-five states currently forbid mandatory union fees, and lawyers for the teachers hope the Supreme Court will extend that nationwide and deal a victory to teachers like lead plaintiff Rebecca Friedrichs.
"A victory for the plaintiffs will challenge unions because it will reduce the resources they have to represent their members since they will have to represent non-members for free," said Professor Ann C. Hodges, a professor of Law at the University of Richmond who signed a brief in support of the unions.
Michael A. Carvin, a lawyer for Friedrichs, argued in court papers that California law "requires thousands of public-school teachers to pay hundreds of millions of dollars" to the teacher's union. He said that the distinction between permissible fees that are supposed to go toward collective bargaining and impermissible fees that go to political ideology is "unworkable" in the public sector. For instance, Carvin said that state law authorizes teachers unions to bargain over class size and teaching tenure, two hotly debated policy issues that are currently playing out in the political debate.
His clients don't want to have to pay fees for union positions that they don't support. "Bedrock First Amendment principles forbid the compelled support of ideological advocacy," Carvin argued.
Carvin wrote the agency fees typically consume about 2% of a new teacher's salary.
Lawyers for the California Teachers Association defend the state system and argue that the agency fees serve the interest of the state to engage in "orderly negotiation of terms and conditions of employment and resolution of employee grievances," while at the same time ensuring the "fair allocation" of the costs that service all employees. They say the vast majority of issues on which the unions bargain are of interest to all union and non-union employees alike.
"Outlawing fair-share fees will override the judgements of 23 States plus the District of Columbia that have enacted statutory collective-bargaining frameworks covering public-education employees," David C. Frederick, a lawyer for the union, argued in court papers. "It will also throw into disarray tens of thousands of collective bargaining agreements governing millions of teachers, police officers, fire-fighters, first responders and others public employees," he said.
The Obama administration supports the unions in the case.
"Petitioners' attempt to demolish this Court's settled framework for analyzing conditions of public employment would astonish the Founding generation and would stamp out the State-by-State variation in public-employment structures that has been the hallmark of this Court's First Amendment jurisprudence for decades," Solicitor General Donald B. Verrilli wrote.
The Court is also considering a second question in the case concerning whether non-members must affirmatively opt out of a union's ideological spending. Critics say that the unions have made it too difficult for nonmembers who object to paying dues to opt out of that funding.