WASHINGTON (CNN) -- Labor unions must give nonmember workers "fresh notice" of unplanned increases in fees or assessments -- money that might be used for political purposes -- the Supreme Court ruled Thursday.

The justices said those nonunion workers must be allowed to make an informed choice about how those kinds of fees are spent, allowing them to object and obtain a rebate.

The 7-2 decision is a victory for Dianne Knox, a California state employee, who sought to opt out of a $12 million assessment imposed by the Service Employees International Union Local 1000. She did not belong to SEIU, unlike most of her fellow government workers.

State law allows unions to charge nonmembers annual dues -- called "fair share fees" -- to help support the collective bargaining efforts on behalf of all government workers, including the majority of workers who belong to the SEIU.

At issue was whether nonunion employees may be constitutionally required to pay special fees to a union to help cover things like political spending on ballot measures.

After two ballot measures were added in a special state election in 2006 , the union decided to impose a 25% supplemental, separate, and temporary increase in fees to "build a political fight-back fund."

The money, said officials, would be used "for a broad range of political expenses, including television and radio advertising, direct mail, voter registration, voter education, and get-out-the-vote activities in our work sites and in our communities across California."

The high court noted nonunion members were not given a choice on whether they would "opt in" and pay into this special fund, and the money was taken directly out of their paychecks and pensions.

About 28,000 current and former workers who were not members of SEIU sued, and the high court supported them.

"This aggressive use of power by the SEIU is indefensible," wrote Justice Samuel Alito. "We see no justification for the union's failure to provide a fresh ... notice" of the special fee, whether it involves political activities or not.

"When a public-sector union imposes a special assessment of dues increase, the union must provide a fresh ... notice and may not exact any funds from nonunion members without their affirmative consent," Alito added.

The high court in past cases has ruled nonunion public sector workers cannot be forced to pay for union activities not directly related to collective bargaining. That would include many kinds of political lobbying and advertising.

Justices Sonia Sotomayor and Ruth Bader Ginsburg agreed with their more conservative colleagues that fair notice must be given for nonmembers to opt out of pure political lobbying by unions. But they said the majority went too far by ruling that the requirement applies to other kinds opt-out fee collections as well, which they said was never argued or briefed by the opposing parties.

In complete dissent were Justices Stephen Breyer and Elena Kagan.

"If the union's basic administrative system does not violate the Constitution, then how could its special assessment have done so," asked Breyer, who delivered his dissent from the bench, a rarely exercised privilege. "I do not believe the First Amendment requires a second objection opportunity to those nonmembers who did not object the first time."

The case is Knox v. Service Employees International Union (10-1121).