Walker, meanwhile, is casting the recall as a battle with out-of-state "union bosses" who want to benefit from taxpayer money.
"This is about sending a message about don't mess with us or we'll take you out no matter who you are," Walker said in an interview with The Associated Press. "For all the talk about collective bargaining, for the national unions it's really about the money."
Walker has already raised more than $12.1 million for the race — the majority of it from out-of-state donors — breaking a previous fundraising record that Walker himself set in the 2010 governor's race. Unions plan to spend at least as much as they spent on the state Senate recall campaign.
Walker's proposal, which passed the Republican-controlled Legislature despite massive protests and all 14 Senate Democrats fleeing to Illinois for three weeks, targeted only public workers and exempted most fire and police officers.
It forced state and local government employees, including teachers, to pay more for health insurance and pension benefits, and stripped away collective bargaining rights except over salary increases no greater than inflation. It also did away with automatic dues withdrawals and forced annual votes for the unions to stay officially recognized.
Hananel reported from Washington, D.C.
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