The money collected from union members has helped make teachers, prison guards and nurses among the most powerful interest groups in Sacramento. They overwhelmingly support Democrats, who control both chambers of the Legislature and every statewide office.
While labor led the No on Prop. 32 fight, businesses and wealthy Republicans funded the opposing campaign.
Prominent supporters include Stanford physicist Charles Munger Jr., the son of billionaire Berkshire Hathaway Inc. Vice Chairman Charles Munger, who has donated more than $10 million, and former Univision chief executive Jerry Perenchio, a frequent Republican donor, who has given $800,000.
More than $4 million came from the American Future Fund, an Iowa-based organization that supports conservative causes with ties to billionaires Charles and David Koch, who have provided crucial support to the tea party.
Other contributions have been more mysterious. The Fair Political Practices Commission filed a successful lawsuit seeking the records of the Phoenix-based Americans for Responsible Leadership after the group made an $11 million contribution to the Small Business Action Committee PAC. The political action committee is supporting a dual campaign to pass Proposition 32 and defeat Gov. Jerry Brown's November tax initiative.
The lawsuit revealed that a secondary group, again with ties to the Koch brothers, was behind some of the out-of-state money.
Art Pulaski, executive-secretary treasurer of the California Labor Federation, which represents more than 2 million members, said the rejection of Proposition 32 represents a victory against right-wing billionaires trying to influence California voters.
"Now that we know that the Koch brothers are behind it, even though they don't admit it, we want to let them know they can't dismantle the democratic voice," Pulaski said in an interview.
Supporters had described Proposition 32 as a cure-all for special-interest politics by prohibiting corporations and unions from making donations directly to state candidates. Email and telephone messages left for the campaign weren't immediately returned early Wednesday.