The proposal comes four years after an identical measure prompted intense debate and an eventual veto from then-Gov. Bill Ritter, a Democrat.
The legislation is anticipated on Wednesday, the first day of Colorado's 120-day lawmaking session.
Democratic Sen. John Morse says he expects the measure to clear the Legislature, which controlled in both chambers by his party.
The bigger question is whether Democratic Gov. John Hickenlooper will approve. Ritter was dogged by firefighter protesters when he vetoed the union bill in 2009. Unions felt betrayed by the man they'd helped elect.
The Colorado Municipal League opposes the measure. Republican Sen. Greg Brophy called the proposal a bad idea that can prove costly for municipalities.
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