The Supreme Court on Thursday handed a setback — and an ominous warning — to a labor giant that has been one of the most generous bankrollers of the super PACs backing President Obama and other Democratic candidates.

At issue in the case: tactics that the Service Employees International Union, which has bankrolled a constellation of Democratic super PACs to the tune of more than $3 million, used to beef up its political warchest in California during 2005. SEIU imposed a special assessment on its members to fight a pair of ballot initiatives aimed at public service unions. The high court ruled that non-union members required to pay fees to work in unionized shops should have been given the chance to opt out of the special assessment. 

Even some of the staunchest liberals on the bench, Justices Sonia Sotomayor and Ruth Bader Ginsburg, agreed with the court's 7-2 decision that the Service Employees International Union was in the wrong when it forced California government employees to pay an "emergency assessment" for a campaign against two anti-union ballot measures without giving non-union members an opportunity to opt out.

But in writing for the majority, Justice Samuel Alito said that from now on, unions will have to gain permission from non-union employees before raising fees or making special assessments. Currently, employees must affirmatively opt out in order to avoid, or reduce their payments.

And Alito made it plain that he believes the court should take more sweeping action to prevent unions from collecting any fees from non-union workers. The California case involved an "agency shop," a relatively common arrangement in which workers who do not join the union are nonetheless required to pay the union a percentage of their salary. Unions argue that this prevents "free riders" from benefiting from labor organizers' work without paying to support it.

In his opinion, however, Alito termed this an "impingement" on First Amendment rights. "Acceptance of the free-rider argument as a justification for compelling nonmembers to pay a portion of union dues represents something of an anomaly," he wrote. "An opt-out system creates a risk that the fees paid by nonmembers will be used to further political and ideological ends with which they do not agree."

Sotomayor, in a separate opinion joined by Ginsburg, concurred with the court's decision but vehemently disagreed with Alito's questioning of "the validity of our precedents" regarding unions' rights to collect fees from workers in union-organized workplaces. She noted that Alito's opinion "strongly hints" at a desire to go beyond fee increases and special assessments to question the agency fees paid by non-union members.

So far this campaign cycle, according to figures available on Sunlight's Follow the Unlimited Money, the SEIU has given $1 million to Priorities USA Action, the super PAC supporting Obama, and nearly more than $660,000 to Majority PAC, focused on Senate races, House Majority PAC and the opposition research group American Bridge 21st Century. All are super PACs started by former top Democratic aides to support the party's candidates.