US President Joe Biden joined striking auto workers on the picket line in Michigan Tuesday in a historic first for a sitting US president, a day before rival Donald Trump makes his own bid for the blue-collar vote in the battleground electoral state.

Wearing a baseball cap with the logo of the United Auto Workers (UAW) union, the 80-year-old Democrat told banner-waving employees through a megaphone that he was on their side.

Republican Trump will visit Michigan on Wednesday, turning the strike into a bitter early confrontation between the two top candidates for an election that is still more than a year away.

Biden told workers that the "Big Three" automakers -- Ford, General Motors, and Stellantis -- were "doing incredibly well, and guess what, you should be doing incredibly well too."

"You deserve the significant raise you need and other benefits," he said to cheers from the crowd.

White House Press Secretary Karine Jean-Pierre described it as a "historic" trip.

"Today will mark the first time a sitting president has visited a picket line in modern times," Jean-Pierre told reporters aboard Air Force One. "This is an important message to America's auto workers."

The UAW's outspoken chief Shawn Fain greeted Biden on the tarmac in Detroit and accompanied him to the picket line.

For Biden, facing concerns about his poll ratings, his age, and the economy, the trip is a golden opportunity to woo working-class workers and union members.

'Huge'

"That is huge," auto worker Patrick Smaller, 56, said about Biden's visit as he stood on the picket line outside a massive Ford plant in Wayne County, Michigan on Tuesday.

"He believes in what we stand for."

As cars and trucks honked in support, another worker, Tiara Conner, said Biden's visit was "great". 

She said she was "not surprised" that Trump was visiting too, adding: "I just hope that he (Trump) is also here for the right reasons and standing in solidarity with us." 

The current and former presidents are both targeting the blue-collar vote in Michigan, a key swing state that Trump won in 2016 and then Biden flipped back in 2020.

But their messages there are very different as they look towards a rematch next year.

Biden has consistently talked up his pro-union credentials, and an endorsement from the United Auto Workers (UAW) union helped him secure the presidency three years ago.

"I always support the UAW", Biden said on Monday.

Trump however will be focusing on winning back the working class voters who helped propel him to the White House in 2016, rather than unions with whom he has long had difficult relations.

'Take your jobs'

Trump has focused on attacking Biden's drive to fund a shift to more environmentally friendly electric vehicles, saying it is driving jobs abroad.

"Remember he wants to take your jobs away and give them to China," Trump wrote on his social media site Truth Social.

Trump also accused Biden of copying his plans and of "pretending" to be a picket.

However, Biden says his push for electric vehicles is part of a plan to bring manufacturing jobs back to the United States and put the country in the lead of a global race to develop green technology.

Jean-Pierre said Biden was "fighting to ensure that the cars of the future will be built in America, by unionized American workers in good paying jobs, instead of being built in China." 

Biden's Michigan trip carries a political risk as he must tread a fine line between backing the workers and trying to end a strike that is costing the economy billions of dollars.

The White House deflected a barrage of questions about whether Biden was taking sides in the dispute, saying the president wanted a "win-win" agreement and would not get involved in negotiations.