'Let's keep going': Biden joins striking autoworkers on picket line

President Joe Biden joined the United Auto Workers picket line Tuesday in his strongest show of support yet for union members striking against General Motors, Ford and Stellantis.

Biden, donning a UAW baseball cap, used a bullhorn to address members striking outside General Motors' Willow Run Redistribution Center in Belleville, Michigan.

"The fact of the matter is that you guys, UAW, you saved the automobile industry back in 2008 and before," Biden said. "Made a lot of sacrifices, gave up a lot and the companies were in trouble. But now they're doing incredibly well and guess what, you should be doing incredibly well too."

"Folks, stick with it because you deserve the significant raise you need and other benefits," he continued.

Biden's appearance is a move some experts are calling apparently unprecedented in modern political history, something the White House has been quick to tout.

"It is a historic visit, and this is an active picket line," White House press secretary Karine Jean-Pierre told reporters aboard Air Force One en route to Michigan.

Biden's trip comes one day before former President Donald Trump arrives in the 2024 battleground state to campaign in lieu of participating in the second Republican primary debate Wednesday night. Trump announced earlier this month he would give a speech to current and former union workers. Jason Miller, Trump's senior adviser, criticized Biden's upcoming trip as a "cheap photo op" and reaction to Trump's plans.

The White House said Monday Trump's schedule didn't factor into Biden's decision to accept an invitation from UAW president Shawn Fain to join members on the picket line.

UAW launched its strike against the "Big 3" U.S. automakers on Sept. 15 after failing to reach a contract agreement. The union, which is conducting a "stand-up" strike, recently expanded its walk outs with 38 new locations targeting Stellantis and GM.

Key among the union demands are a 36% pay increase over four years and a 32-hour work week.

The White House has repeatedly declined to say if Biden, who touts himself as the most pro-union president, supported those specific requests.

"I'm not gonna get into the details of what's being negotiated right now on the table with certainly with the parties," Jean-Pierre said when pressed on that point by ABC News Senior White House Correspondent Selina Wang during Monday's briefing. "What we're saying is that we support the autoworkers."

Biden previously said he believes union workers deserve their "fair share" of the record earnings of the companies, and that he hoped both sides could come to a "win-win" agreement.

"Unions built the middle class, that's a fact," Biden told UAW members on Tuesday. "So let's keep going. You deserve what you've earned and you earned a hell of a lot more than you're getting paid now."

After his brief remarks, Biden was asked by a reporter if workers should get a 40% pay increase. Striking members around him immediately responded yes, and he appeared to say "yes" as well -- which would be the furthest Biden has weighed in on that specific union demand.

His visit to the picket line, however, is not without political risk for Biden as he runs for reelection.

He originally tapped two top officials -- acting Labor Secretary Julie Su and senior adviser Gene Sperling -- to offer their support to both parties in reaching agreement. Biden's announcement last Friday that he would join the picket line came hours after UAW's Fain publicly extended an invitation.

Fain stood with Biden at the picket line and thanked him for "being a part of this fight."

"Thank you Mr. President for coming to stand up with us in our generations-defining moment," Fain said. "And we know the president will do right by the working class and when we do right by the working class, you can leave the rest to us because we are going to take care of this business."

ABC's "This Week" co-host Martha Raddatz asked Transportation Secretary Pete Buttigieg whether Biden joining the picket line is a "good idea."

"That's where he stands," Buttigieg responded. "He's a pro-worker president. He is an unapologetically pro-union president. And, you know, not just in contrast to the anti-union policies of the Trump administration, but really with respect to presidents of both parties over the last half century. He's proud of the fact that he is the most pro-union and pro-worker among them. And, by the way, getting this right."

Buttigieg also said he believed the companies and auto sector will "benefit in the long run" from the deal.

Asked if Biden had spoken recently with the three auto companies, Jean-Pierre said she had no calls to read out to reporters.

ABC News' Justin Gomez and Molly Nagle contributed to this report.