A judge ruled this week that the Service Employees International Union improperly coerced workers caught in the middle of SEIU’s high-stakes turf battle with a breakaway union in California, potentially invalidating a 2010 election involving 43,500 employees.

SEIU, the nation’s most politically influential union, has been engaged in a costly fight with the former leaders of a 150,000-worker California chapter that formed a breakaway union in 2009. The split followed clashes with then-SEIU President Andy Stern over his emphasis on growing membership even if it meant giving concessions to employers.

Last fall, SEIU won the biggest standoff, an election to represent 43,500 Kaiser Permanente workers in Northern California.

The vote was a big setback for the breakaway union, the National Union of Healthcare Workers, leaving it with fewer than 10,000 members. But this week, Administrative Law Judge Lana Parke ruled that Kaiser had improperly withheld pay raises from workers in Southern California who had switched to the new union and that SEIU had then improperly threatened the workers voting in the Northern California election that they, too, could have raises denied if they made the switch.

It is now up to the National Labor Relations Board to decide whether to call a second election, as the judge recommends.

Leaders of the breakaway union noted that the ruling came at the same time as SEIU and other unions are arguing in favor of new rules proposed by the labor relations board to reduce employer coercion against workers before union elections.

“SEIU has been promoting itself an as advocate for labor law reform and workers, and against coercion and intimidation, but no institution has done more to coerce and thwart workers about which union they want to join,” said John Borsos, vice president of the breakaway union.

Dave Regan, who is leading the fight for SEIU in California, said in a statement that SEIU would prevail. “If a new election is run we will defeat them by a larger margin than last time,” he said.