During his two decades building one of the largest union locals in California, Sal Rosselli earned a reputation as a cunning strategist and street fighter — someone who often vilified hospital chains during contract battles.
These days, he is using those brass knuckles on his former colleagues at the Service Employees International Union in a battle that threatens to rip a giant hole in the most powerful union in the nation’s largest state.
The S.E.I.U.’s national leadership ousted Mr. Rosselli last year after a power struggle that ended with a jury finding that he had improperly used member dues to form a breakaway union. Shortly after being ousted, Mr. Rosselli did create a rival union, and now he is trying to lure many of his former members — 43,000 workers at Kaiser Permanente, the largest health care provider in the state.
On Monday, workers at 331 Kaiser facilities across California began voting by mail on whether to bolt the S.E.I.U. and join Mr. Rosselli’s group, the National Union of Healthcare Workers.
A victory would give a vital boost to Mr. Rosselli’s fledgling 6,000-member union, all but assuring its long-term survival. It would also be a huge blow to the 1.9-million-member service employees union, since Mr. Rosselli’s group would gain the stature and dues money to finance a broader war for far more S.E.I.U. members.