In 14 years atop the SEIU, Mr. Stern pioneered a new labor strategy: Size brings power, so grow at all costs. He bullied and charmed companies to get them to unionize—often, claimed his critics, in exchange for weak contracts for the workers. He raided smaller unions. And he centralized power around the group’s Washington headquarters. In a decade, the SEIU doubled its membership and became the single biggest contributor to the Democratic Party.