Police used pepper spray to subdue protesters trying to rush Michigan's state senate after the governor and other Republican leaders said they would press for quick approval of right-to-work laws limiting union powers.
The Michigan house of representatives voted 58-52 on Thursday night to approve the legislation banning most private unions from collecting fees from non-union employees. The senate also approved the laws by a six-vote margin.
Opponents of the right-to-work measures say they drain unions of money and weaken their ability to bargain for good wages and benefits. Supporters insist it would boost the economy and job creation.
A victory in Michigan would have given the movement its strongest foothold yet in the industry-heavy Rust Belt region, where organised labour has already suffered several body blows.
Wisconsin's Republican governor Scott Walker pushed through a law last year that effectively ended collective bargaining rights for most public sector workers, leading to huge protests at the state capitol and an effort to recall the governor.
Mr Walker survived the recall referendum in June and a court battle over the law's constitutionality is continuing.
Even before the votes in Michigan, protesters streamed inside preparing for what appeared inevitable after governor Rick Snyder joined Republican legislative leaders on Thursday morning in announcing they would push for swift passage.
Eight people were arrested for resisting and obstructing when they tried to push past two troopers guarding the senate door, state police inspector Gene Adamczyk said.
After repeatedly insisting during his first two years in office that right-to-work was not on his agenda, Mr Snyder made a U-turn on Thursday, a month after voters defeated a ballot initiative that would have barred such measures under the state constitution.
"This is all about taking care of the hard-working workers in Michigan, being pro-worker and giving them freedom to make choices," Mr Snyder said during a news conference. "The goal isn't to divide Michigan, it is to bring Michigan together."