(CNN) - Members of the United Auto Workers union working at General Motors decided Saturday night to stay on the job past a midnight deadline for a new contract, but remained on the verge of a strike.
In letters late Saturday to both GM and 46,000 hourly workers at 31 GM factories and 21 other facilities across the nation, the union said no strike would be started at least until a Sunday 10 a.m. ET meeting of union officials representing members at GM.
While there had been some progress in talks, there remained "significant differences between the parties on wages, health care benefits, temporary employees, job security and profit sharing," Terry Dittes, the union vice president leading the negotiating team, told membership.
Those issues suggest it will be difficult to reach an agreement in time to avoid a strike, said Kristin Dziczek, vice president of industry labor and economics for the Center for Automotive Research, a Michigan think tank.
"You can't solve one of those issues without solving all of them," she said.
If the union goes on strike, it will be the first such action against a US automaker since 2007 and the largest against any US business in that time.
Dittes in a statement accused GM of refusing "to put hard working Americans ahead of their record profits of $35 billion in North America over the last three years."
"We are united in our efforts to get an agreement our members and their families deserve," he added.
GM, the largest US automaker, said in a statement that it is willing to work around the clock to try to reach a deal to build "a strong future for our employees and our business."
"We continue to work hard on solutions to some very difficult challenges," said the company's statement. It said "there are thousands of GM families and their communities -- and many thousands more at our dealerships and suppliers -- counting on us for their livelihood."
The union had earlier extended the contracts at two other US automakers with UAW contracts, Ford and Fiat Chrysler, as it targeted GM in an effort to reach a deal that would set a pattern for the industry. Dittes' letter said that while membership will keep working for the time being, there would be no long-term extension of the contract at GM.
Talks were continuing Saturday evening leading up to the midnight deadline. But Dittes' letter to GM said that the union's executive board would meet at midnight, ahead of the separate union meeting scheduled later on Sunday morning.
All three automakers are dealing with slower sales and the need to make huge multi-billion-dollar investments in developing electric and self-driving vehicles that have more long-term potential than current market demand.
To save money for those efforts, GM has already halted operations at three US plants -- two transmission factories and an assembly line in Lordstown, Ohio. It plans to shut another assembly line in Detroit, its last Detroit factory, early next year.
The UAW has vowed to win GM's agreement at the negotiating table to keep all or at least some of those plants operating.
But negotiations took place as the union is hit by a scandal involving misappropriation of union funds, and in some cases, union officials accepting bribes from officials at Fiat Chrysler. Nine people associated with the union or Fiat Chrysler have already pleaded guilty to federal charges.
Earlier in the week, the Detroit News reported the union's president, Gary Jones, was the unnamed union official identified in the most recent indictment as "UAW Official A." The union has not responded to a request for comment about that report.
Experts say the scandal will make it more difficult to get rank and file union members at the automakers to ratify any tentative deal reached by union leadership. Four years ago the deals all passed by only narrow margins, even though there was no scandal at that time.
Dziczek said the two meetings of union leadership scheduled for Sunday is unusual at this stage of negotiations. It could be a sign of the the strain that the scandal is putting on the union leadership during these talks.
"It insures whatever is decided, there is buy-in. And they need that," she said.
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