Biden to join striking auto workers on picket line in move White House calls 'historic'

President Joe Biden will join 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 announced Friday on X that he would travel to Wayne County, Michigan, "to join the picket line and stand in solidarity with the men and women of UAW as they fight for a fair share of the value they helped create."

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

"He supports the UAW workers and tomorrow what you're going to see is historic," White House press secretary Karine Jean-Pierre told reporters Monday, although she declined to give further details on where exactly Biden will be, exactly what he'll do and what the trip will entail. "This is going to be a historic visit."

At the same time, Jean-Pierre was peppered with reporter questions about whether the president is officially siding with the striking workers' demands by joining the picket line, and how his appearance is consistent with the White House claim he's not getting involved in the negotiations.

"He is standing with the workers," she said. "We are not involved in negotiations. That is something for them to decide what is going to work for the parties that are involved, but he is standing with the auto workers."

Biden's appearance will come one day before former President Donald Trump arrives in Michigan -- a 2024 battleground -- 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.

But Jean-Pierre during Monday's briefing repeatedly declined to say if Biden, who touts himself as the most pro-union president, supported those specific requests when asked by ABC News Senior White House Correspondent Selina Wang.

"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 replied. "What we're saying is that we support the auto workers," adding at one point that he "stands by the side of the workers."

Biden has said he believes union workers deserve their "fair share" of the record earnings of the companies, he's added that it's time for a "win-win agreement" between the union and automakers.

ABC's Selina Wang also pressed President Biden on whether he supports the UAW demands later Monday afternoon.

“I think the UAW gave up an incredible amount back when the automobile industry was going under. They gave up everything from their pensions on, and they saved the automobile industry,” Biden responded.

“And I think that now that the industry is roaring back, they should -- they should participate in the benefit of that," Biden continued. "And if you take a look at the significant increase in salaries for the executives and growth for the industry, they should benefit from it. So yes, I support -- I always support the UAW."

The move 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.

"We invite and encourage everyone who supports our cause to join us on the picket line from our friends and families all the way up to the President of the United States," Fain said in a video. "We invite you to join us in our fight."

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's Molly Nagle contributed to this report.