LANSING, Mich. (AP) -- The prospect of an auto workers strike could test Joe Biden 's treasured assertion that he's the most pro-union president in U.S. history.

The United Auto Workers is threatening to strike against the nation's big three automakers, General Motors, Ford and Stellantis, if tentative contact agreements aren't reached by 11:59 p.m. on Thursday. That could reshape the political landscape in the battleground state of Michigan and potentially unleash economic shockwaves nationwide.

The auto industry accounts for about 3% of the nation's gross domestic product and as many as 146,000 workers may walk off their jobs. While the effects would be most immediate in Michigan and other states with high concentrations of auto jobs, such as Ohio and Indiana, a prolonged strike could trigger car shortages and layoffs in auto-supply industries and other sectors.

"Anything that goes beyond a week, you're going to start feeling the pain," said Marick Masters, a business professor at Wayne State University in Detroit. "And anything beyond two weeks, that's when the effects start to compound."

Doc Killian, who has worked in a Ford assembly plant in Wayne, Michigan, for 26 years, says he can no longer afford the cars he helps build, crystallizing how the nation's middle class has been squeezed.

"I think the American public as a whole realizes the impact that the American auto workers have on the economy," Killian said. "If we suffer, the American economy suffers."

Biden has built his political career around just such an argument, repeating the mantra that the "middle class built America, and that unions built the middle class." His administration also has championed organized labor and promoted worker organization unabashedly, with Biden frequently proclaiming himself "the most pro-union president in American history."

Still, Shawn Fain, who was elected president of the United Auto Workers in March after promising a more confrontational stance in negotiating with automakers, countered Biden's claim on CNN this week, saying, "I think there's a lot of work to be done in that category."

The UAW chief has also sought to broaden his argument beyond just auto workers. He said on a livestream with union members that the UAW's demands are about "raising the standard for workers everywhere."

"I truly believe that all of America will stand with us in this fight," Fain said.

Biden also must contend with blunt criticism from former President Donald Trump, the early leader in next year's Republican presidential primary, who is now pushing for the UAW to endorse him -- an unlikely prospect, according to union leaders.

Trump posted online that the "once fabled" UAW "will soon go OUT OF BUSINESS" if Biden "is allowed to pull off his ALL ELECTRIC CAR HOAX. China will build them all. ENDORSE TRUMP!" In another post, the former president appealed directly to rank-and-file union members whose support helped him win Michigan in 2016: "Union leadership must decide whether they will stand with Biden and other far-left political cronies in Washington, or whether they will stand with front-line autoworkers and President Trump."

That referenced new federal rules pushed by the Biden administration requiring two-thirds of new passenger cars sold in the United States to be all-electric by 2032. Trump argued those moves would "murder the U.S. auto industry and kill countless union autoworker jobs forever, especially in Michigan and the Midwest."

But some union leaders and members have scoffed at suggestions that the U.S. not embrace efforts to reduce greenhouse gas emissions since manufacturers in China and elsewhere could rush in to produce electric vehicles if the U.S. doesn't. Fain, who has previously applauded the "transition to a clean auto industry" as long as autoworkers "have a place in the new economy," said Trump was "not someone who stands for a good standard of living."

Dave Green, a UAW regional director in Ohio and Indiana, said the former president "carries no credibility in my book" since "he did nothing to support organized labor except lip service."

Green said he still considers Biden the most pro-union president of his lifetime. But he hopes the White House won't stay neutral if there's a strike.

"We don't forget," Green said. "When you're in distress, the people who are there supporting you -- that goes a long way."

Biden faced some criticism from labor groups last year when he urged Congress to approve legislation preventing rail workers from going on strike, fearing an upending of supply chains heading into the holidays. But, unlike with rail and airline workers, the president doesn't have the authority to order autoworkers to stay on the job.

Nowhere will the political fallout of an auto workers strike be felt more than Michigan, which Biden won by nearly 3 percentage points in 2020. The state shifted further during last year's midterms, leaving the governor's office and Legislature Democratic-controlled for the first time in 40 years.

Michigan since became the first state in nearly six decades to repeal "right to work" laws limiting union activity that had been approved by the GOP-controlled Legislature in 2012. Still, a strike could shake up politics statewide.

"The UAW is a major player in Michigan politics and if there is a strike, of whatever duration, it'll have a political impact," said Mark Brewer, former chairman of the Michigan Democratic Party. A strike, Brewer said, would leave Biden having "to speak and act consistent with his previous advocacy for working people."

That might mean alienating other allies, though, since Biden has in the past received support from top U.S. automakers on the administration's rules over future sales. And Ray Curry, the former UAW president who was unseated by Fain, had worked with Biden in the past, even attending White House ceremonies.

Biden was nonetheless anxious to meet Fain given the pair's shared working-class backgrounds, and they sat down together one-on-one in the Oval Office in July. The White House says it has been in regular touch with the UAW since then, and that overall communication is much better now.

"We are engaged regularly with the parties, and of course seek to support negotiations in any way that is helpful," said Michigan native and longtime Democratic and Biden adviser Gene Sperling, who the president tapped as the administration's point person on the autoworker negotiations. "But there is no substitute for the parties staying at the table 24/7 to come to what the president wants to be a win-win agreement."

Union support was instrumental in helping Biden overcome a slow start to clinch the 2020 Democratic presidential nomination, and it helped him win not just Michigan but Wisconsin and Pennsylvania as he defeated Trump in that year's general election.

Underscoring his commitment to organized labor, Biden's lone campaign rally since launching his reelection bid in April came in June in Philadelphia, when more than a dozen of the country's largest and most powerful unions endorsed Biden for a second term.

So many unions banding together for an unprecedented joint endorsement so early in the election cycle was meant as a show of strength for the president. Conspicuously absent from the event, though, was the UAW. Fain has since said that if Biden wants the UAW's 2024 endorsement, he'll have to earn it.

Other union leaders acknowledged what's at stake for the president.

"Are strikes uncomfortable for an administration?" said Randi Weingarten, president of the American Federation of Teachers, which endorsed Biden's reelection this summer. "Of course they are."

But, she said, "The administration believes in workers and believes that workers have the power to have a better life through collective organization and through collective bargaining."

"This is not a soundbite to them," Weingarten said. "This is a belief system."

___

Weissert reported from Washington. Associated Press writer Tom Krisher in Detroit contributed to this report.