Madison was inundated with an estimated combined 60,000 protesters for the two sides, the Chicago Tribune reported.
Union supports beat drums and chanted "Kill the bill!" while the counter-demonstrators responded with calls for Democratic lawmakers to "Do your job!"
Democrats in the state Senate have at least temporarily blocked a vote on a bill that would strip public employee unions of collective bargaining rights by disappearing out of state. Sen. Jon Erpenbach, a Democrat, reached out to Republican Gov. Scott Walker, suggesting the unions would be willing to concede on wages and benefits if the governor gave in on collective bargaining, the Milwaukee Journal Sentinel reported.
"Gov. Walker has repeatedly said that we won't negotiate the budget and we can't balance the budget on a hope and a prayer," a spokesman, Cullen Werwie, responded.
"That remains true. State and local government need the flexibility to manage this and future budget crises. In addition, as government workers pay a modest amount toward their pension and healthcare premium, about half the national average, it is fair to give them the choice of additional savings on their union dues."
Demonstrators against and for Walker's bill packed the area around the Statehouse and the Capitol Rotunda. Police were brought in from 40 local, county and state agencies.
Police said the protests were generally peaceful with no arrests reported after several hours. The Tribune said the two sides melded at some points, with some engaging in calm debate while others ratcheted the tension up to screams.
The Tribune noted similar legislation is being put forth in Ohio and Iowa with anti-union forces trying other avenues for limiting collective bargaining in Michigan and Indiana.
"This is where we're gonna start," Tea Party organizer Melvin Timm of Neenah, Wis., said. "This is gonna set the tone."
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