More than 49,000 members of the United Auto Workers walked off General Motors factory floors or set up picket lines early Monday as contract talks with the company deteriorated into a strike.
Workers shut down 33 manufacturing plants in nine states across the U.S., as well as 22 parts distribution warehouses.
It wasn't clear how long the walkout would last, with the union saying GM has budged little in months of talks while GM said it made substantial offers including higher wages and factory investments.
It's the first national strike by the union since a two-day walkout in 2007 that had little impact on the company.
GM workers joined striking Aramark-employed janitors on the picket lines Sunday night at a sprawling factory on the border between Detroit and the small town of Hamtramck.
Worker Patty Thomas said she wasn't scheduled to picket, but came out to support her colleagues at the car plant, which GM wants to close.
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She's heard talk that GM may keep the factory open and start building electric pickup trucks there, but she's skeptical.
"What are they going to take away?" she asked. "That's the big issue." She said workers gave up cost-of-living pay raises to help GM get through bankruptcy, and workers want some of that back now that the company is making profits.
Striking GM employees were joined on the picket lines by workers from Ford and Fiat Chrysler, who are working under contract extensions.
Night shift workers at an aluminum castings factory in Bedford, Indiana, that makes transmission casings and other parts shut off their machines and headed for the exits, said Dave Green, a worker who transferred from the now-shuttered GM small-car factory in Lordstown, Ohio.
Green, a former local union president, said he agrees with the strike over wages, plant closures and other issues.
"If we don't fight now, when are we going to fight?" he asked. "This is not about us. It's about the future." UAW Vice President Terry Dittes, the union's top GM negotiator, said a strike is the union's last resort but is needed because both sides are far apart in negotiating a new four-year contract. The union, he said Saturday, does not take a strike lightly.
"We clearly understand the hardship that it may cause," he said.
"We are standing up for fair wages, we are standing up for affordable quality health care, we are standing up for our share of the profits."