Trump and Biden go back-and-forth over swing state union's endorsement

After the United Auto Workers union officially endorsed President Joe Biden last week, former President Donald Trump lashed out at the union's president on Sunday, prompting the Biden campaign to bite back -- as the two campaigns vie for support with union voters, who have been influential in some past elections.

The heated back-and-forth over the endorsement from the UAW, based in the battleground state of Michigan, comes as both Biden's and Trump's 2024 teams say they're eager for a rematch in November.

The Biden-Harris campaign, in a statement on Sunday night, slammed what they called Trump's "wounded ego" after the former president attacked the UAW's president, Shawn Fain, on social media.

"So…apparently losing the UAW endorsement to Joe Biden has left Donald Trump's wounded ego with quite the SCAB," said communications director Michael Tyler.

The statement followed Trump calling Fain a "STIFF" in a lengthy social media post in which he also accused the union leader of "selling" the U.S. automobile industry to China and blasted Biden's support for electric vehicles.

"Shawn Fain doesn't understand this or have a clue," Trump wrote. "Get rid of this dope & vote for DJT. I will bring the Automobile Industry back to our Country."

Fain, earlier in the day, had touted Biden's support for unions in an interview with CBS' "Face the Nation" and said that he "can't fathom any union supporting Trump."

"When you look at these two candidates, you know, Joe Biden has a history of serving others and serving the working class and fighting for the working class, standing with the working class. Donald Trump has a history of serving himself and standing for the billionaire class. And that's contrary to everything that working class people stand for," Fain argued, hours before Trump's post.

Biden, who won the UAW's endorsement in 2020 as well, has repeatedly called himself the "most pro-union" president in history and last September became the first modern president to join a picket line, with auto workers in Detroit.

Trump has attacked Biden's union record as "nonsense."

Exit polling in the 2016 race showed former Secretary of State Hillary Clinton faltered with union workers in Ohio and Michigan. Trump beat her in both.

In 2020, Biden narrowly won back Michigan and two other Rust Belt states, Pennsylvania and Wisconsin, that Trump won four years earlier.

Fain made the UAW's 2024 endorsement official last week at a conference in Washington, D.C., where he also went after Trump in a sharply worded speech.

"Donald Trump is a scab," he said on Wednesday. "Donald Trump is a billionaire, and that's who he represents. If Donald Trump ever worked in auto plant, he wouldn't be a UAW member -- he'd be a company man trying to squeeze the American worker."

ABC News' Lalee Ibssa, Soo Rin Kim and Selina Wang contributed to this report.