WASHINGTON — In its line of cases dealing with the tension between the free-speech rights of unions and the same rights of dissident nonmembers, the Supreme Court has long given considerable deference to the unions.

In decisions ranging from Abood v. Detroit Board of Education in 1977 to Lehnert v. Ferris Faculty Association in 1991, it has said unions should be able to collect so-called agency fees, equivalent to union dues, from nonmembers, because much of what the union does benefits members and nonmembers alike in a given workplace.

While giving credence to concerns about labor peace and the problem of non-paying “free riders,” the Court has also added a caveat: There should be a mechanism whereby nonmembers can decline to finance a union’s political activities. But based on the premise that “dissent is not to be presumed,” it has allowed the unions to collect the full fee from nonmembers unless the nonmembers “opt out.”

Yesterday, it seemed likely that the new Roberts Court is poised to make a dramatic shift in its agency-fee doctrine by upholding a Washington state law that presumes a nonmember does not want his or her money spent for union political activities unless he or she affirmatively “opts in” and allows such use.

In questions and comments during oral argument in Davenport v. Washington Education Association and Washington v. Washington Education Association, justices showed far more concern about the rights of nonmembers than about the rights of unions. Justice Stephen Breyer appeared to be the only one of the nine justices sympathetic to the unions.

“Why should the First Amendment require anything other than an opt-in scheme?” the Court’s newest justice, Samuel Alito, asked at one point, exemplifying the new tone.

The arguments cheered conservatives, including the National Right to Work Legal Defense Foundation, which has long battled against union-fee arrangements that it says stifle the rights of nonmembers.

“I have been attending arguments on these issues for 25 years, and in all of them, so many justices were not very sympathetic toward nonmembers,” said Glenn Taubman, a senior attorney with the National Right to Work Legal Defense Foundation. “Today it was completely the opposite.”

As Taubman’s comment suggests, the case has symbolic and political significance for union and individual rights that may exceed the actual dollars involved.

In contrast to the 70,000 members of the Washington Education Association, there are only about 4,000 nonmembers who pay agency fees, and just a small portion of the dues go toward political activities. But unions are strenuously fighting the “opt-in” alternative, which they fear will result in far fewer teachers giving support to unions’ First Amendment activities. Other states are eyeing Washington’s law, and if it is upheld, some may follow suit.

Alito, who has usually been a relatively quiet and infrequent questioner, repeatedly joined in the argument yesterday, suggesting that he could not fathom why a nonmember would ever want to pay the portion of dues that goes toward political activities.

“What would be the thinking of such a person?” said Alito. “Why would I choose to give up the benefits of union membership and yet want to allow the union to spend my money for its political purposes?”

John West, arguing for the union, replied that it was very possible that a nonmember would still want to support the union’s lobbying for cost-of-living adjustments for teachers or to reduce class size.

Justice Anthony Kennedy also appeared critical of the union stance, telling West, “You proceed as if there are no First Amendment rights of (nonmember) workers.” West answered that nonmembers do have rights that are “fully protected” by the old opt-out process.

But then Justice David Souter asked, “Why can’t the state protect it more?” referring to the First Amendment interests of nonmembers.

Justice John Paul Stevens also seemed to doubt that a nonmember’s failure to opt out of the dues payment in a timely fashion could be read as a surrender of his First Amendment right not to participate in the union’s political activities.

Washington Attorney General Robert McKenna defended the “opt-in” provision as a way of protecting the integrity of elections by ensuring that only those who choose to pay for union political activities do so.

Solicitor General Paul Clement, supporting the state of Washington, asserted that the “opt-in” statute “does not limit the union’s ability to spend its own money on political causes,” so it does not infringe on the union’s free-speech rights.