Labor unions and business interests have been quietly raising millions of dollars and testing campaign messages for months, girding for a brawl over a November ballot measure that could fundamentally shift political power in Sacramento. Now, on the heels of an election that saw unions handed a major defeat last week in Wisconsin, the opposing camps in California soon will launch a campaign battle likely to consume $50 million or more in political spending. "Unions have just two channels of influence," said Daniel J.B. Mitchell of the Anderson Graduate School of Management at UCLA, "collective bargaining and the political side,...