Detroit, Michigan - High-wire talks between striking US workers and automotive giants are in a "critical phase," Jeep-maker Stellantis said Saturday, as politicians staked out positions on a labor issue that could have national impact.

Stellantis, together with fellow "Big Three" automakers General Motors and Ford, was hit Friday by a limited strike – but one the United Auto Workers (UAW) warns could spread.

Both sides issued cautious statements Saturday.

"Our bargaining team continues to work days, nights and weekends" in pursuit of a "reasonable" solution, said the statement from Stellantis, which was formed by the merger of Fiat Chrysler and the French PSA Group.

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But the automaker warned that if talks took a bad turn, the outcome "will take us backward and endanger the long-term competitiveness of our Company, negatively impacting our workers and our communities."

There was no immediate formal response from the union, but a UAW source told AFP, "We had reasonably productive conversations with Ford today."

Only about 12,700 of the UAW's 150,000 members are currently on strike.

But with workers at all of the Big Three coordinating strike action for the first time – including a demand for pay increases of 40% over a four-year contract – the automakers could face a far more disruptive stoppage.

Underscoring the political stakes of the moment, President Joe Biden quickly lent his support to the strikers Friday, saying he understood their "frustration."

On Saturday, former President Barack Obama lent his backing, with a reference to the 2008-09 financial crisis.

"When the big three automakers were struggling to stay afloat, my administration and the American people stepped in to support them," he said on social media. "So did the auto workers in the UAW who sacrificed pay and benefits to help get the companies back on their feet."

"Now that our carmakers are enjoying robust profits, it’s time to do right by those same workers."

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Democratic Senators Gary Peters, from Michigan, and John Fetterman, from Pennsylvania, posted pictures on social media Saturday of the two of them joining UAW workers on a picket line in Michigan.

"Welcome to Michigan @SenFettermanPA," Peters wrote on X, formerly known as Twitter. "Great to have you join us... standing in solidarity."

"I will always have your back," Fetterman posted, saying it was an "honor" to walk with the strikers.

Senator Bernie Sanders of Vermont held a rally in support of UAW workers in Detroit on Friday, while 2024 Green Party presidential candidate Cornel West has also issued a statement of support.

But former president Donald Trump, who hopes to become the 2024 Republican nominee, lashed out at the UAW as over-reaching.

"The auto workers will not have any jobs... because all of these cars are going to be made in China – the electric cars, automatically, are going to be made in China," he said in an interview to be aired Sunday on NBC's Meet the Press.

In its statement, Stellantis said the UAW was misrepresenting its proposals. It said its current offer would give employees a 21% pay raise over the term of the contract, with 10% coming upon ratification.

General Motors upped its offer Thursday, lifting a proposed wage increase from 18 to 20%, according to the UAW.

But hourly workers say the auto giants must produce significantly better packages to make up for what they call meager wages and benefit cuts after the 2008 financial crisis, when both GM and Chrysler, now part of Stellantis, underwent bankruptcy reorganizations.

They also want pay boosts for lower-paid temporary workers.