New York (CNN Business) The United Auto Workers union said Sunday that its members at General Motors will walk out by midnight if the automaker does not meet its demands, setting the stage for the nation's first auto strike in 12 years.

A union statement suggests the two sides are still very far apart in negotiations for a new contract.

As union officials met in Detroit Sunday morning, the union issued a new strike threat.

"Ifrefuses to give even an inch to help hard-working UAW members and their families then we'll see them on the picket lines tonight," said the statement.

The union's contract with GM had expired at 12:01 a.m. Sunday but the union's 46,000 members at GM did not walk off the job at 31 GM factories and 21 other facilities across the nation at that time.

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 in a letter late Saturday.

The company on Sunday issued a statement detailing its most recent offer, including 5,400 jobs and $7 billion in investment at its US plants during the life of the contract. It also promised wage or lump sum pay increases in all four years of the deal, an improved profit sharing formula and a signing bonus of $8,000 per member.

GM said it also offered "solutions" for the two assembly plants that are poised to be closed, one in Lordstown, Ohio, where work stopped earlier this year, and the Hamtramck plant in Detroit, which is scheduled to shut in early 2020. The company did not release details of what those solutions would be.

The UAW has vowed to win GM's agreement at the negotiating table to keep all or at least some of those plants operating.

A person familiar with GM's offer said it included a promise to build a new electric truck at Detroit Hamtramck, and to build new batteries for electric vehicles in Lordstown. That work wouldn't start immediately, so the plants would likely remain dark for some time. Work would start sometime in the next four years if the offer were accepted.

"We presented a strong offer that improves wages, benefits and grows US jobs in substantive ways and it is disappointing that the UAW leadership has chosen to strike at midnight tonight," said GM's statement. "We have negotiated in good faith and with a sense of urgency. Our goal remains to build a strong future for our employees and our business."

Dittes 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.

Those statements from the two sides 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.

"These are some pretty far apart positions. It would take quite a bit of work to bring them together," she said. "It's hard to think they solve any of those issue without solving all of those issues."

If the union goes on strike, it will be the largest by any union against any US business since the last time UAW members struck GM in 2007.

Ford F Fiat Chrysler FCAU The union had earlier extended the contracts at two other US automakers with UAW contracts,and, 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.

There was a strike that started early Sunday morning by 850 UAW maintenance workers at GM plants. But they work for an outside contractor, Aramark, not GM. They had been working under an extended contract since March of 2018.

"We have UAW members who work long, hard hours and are still on public assistance," said Gerald Kariem, Director of UAW Region 1D, speaking of the maintenance workers. "It's shameful."

The UAW members at the three automakers have much better contracts. GM says its average hourly employee earns about $90,000 per year, not including benefits.

All three automakers are dealing with slower sales and the need to make huge multi-billion-dollar investments in developing electric and self-drivin g vehicles that have more long-term potential than current market demand.

It was the need to save money for those efforts that GM halted operations at three US plants -- including the assembly line in Lordstown , and announced plans to shut the Hamtramck plant, its last Detroit factory, early next year.

But negotiations come 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.

Last 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.