SACRAMENTO, Calif.—California Republican interests have qualified a ballot measure that would severely curb the political influence of public and private employee unions while depriving Democratic political candidates of a major source of campaign cash. The measure, which bans political contributions through payroll deductions, qualified Tuesday for the November 2012 statewide ballot. With labor's primary funding source under attack, the measure sets up an expensive campaign season ahead for both sides. Funded in part by wealthy GOP donor and Stanford University physicist Charles Munger Jr. and backed by former Secretary of State George Shultz, the measure bans corporation and union contributions...