About 4,000 workers were off the job at a General Motors plant early Wednesday as a third ongoing strike threatened supplies of popular GM and Chrysler models at the beginning of the spring selling season.

Local 594 of the United Auto Workers went on strike at the Pontiac East plant near Detroit after bargainers failed to reach agreement on a local contract by the 11:59 p.m. Tuesday deadline. Talks were to resume Thursday morning.The plant assembles Chevrolet C/K and GMC Sierra full-size extended-cab pickups, among the company's biggest moneymakers.

The union has demanded more staff.

"There aren't enough people working here," Jim Abare, sourcing coordinator for Local 594, told the industry publication Ward's Automotive Reports. "Workers with 25 years' seniority can't take a vacation day. The company wants to hire temporary workers, but our position is that they should be permanent with full benefits."

GM's Oklahoma City plant, which produces the redesigned Chevrolet Malibu and Oldsmobile Cutlass, has been shut down since April 4 because of a similar dispute over staffing levels.

At Chrysler, the strike at the Mound Road engine plant in Detroit is largely over plans to outsource production of drive shafts to a nonunion supplier. That strike began April 9 and has led to layoffs at 16 other Chrysler plants in North America, affecting 22,981 workers.

The strike has stopped Chrysler's North American production of its Dodge Ram and Dakota pickups and Jeep Grand Cherokee sport utility vehicles. The strike also has halted assembly of the Dodge Viper sports car and Ram vans.

"If they don't settle that strike soon, it will be devastating," said Alan Helfman, general manager of a Jeep-Eagle dealership in Houston. "We're going into our big season."

David Healy, an analyst with Burnham Securities Inc., estimates the strike has cost Chrysler more than $100 million after taxes. He estimates the Oklahoma City strike has cost GM more than $150 million.

While GM has faced a series of strikes in the past two years, this is the first walkout at a Chrysler plant in nearly three years and the longest in more than 30.