US President Joe Biden began an unprecedented trip to join striking auto workers on the picket line in Michigan Tuesday, a day before rival Donald Trump makes his own bid for the blue collar vote in the battleground electoral state.

In what is believed to be a first for a sitting American president, the Democrat will show off his pro-union credentials by standing with employees who have walked out of Detroit's "Big Three" carmakers. 

Republican Trump will then 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.

For the 80-year-old 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.

"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." 

- 'Historic' -

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, saying he would "stand in solidarity" as the union seeks a "fair share" of profits from Ford, General Motors and Stellantis.

The White House said Biden's trip to the picket line would be "historic" and was "absolutely not" influenced by Trump's planned trip the next day.

"This will underscore that the president is the most the most pro-union president in history," Press Secretary Karine Jean-Pierre told a briefing on Monday.

Trump will not be targeting unionized workers when he speaks in front of a plant that makes car parts on the other side of Detroit from where Biden is speaking. 

The former property tycoon, who's skipping a Republican debate to go to Michigan instead, has long wooed working-class voters but has had a difficult relationship with the unions. 

- 'Take your jobs' -

Instead, Trump has focused on attacking Biden's drive to fund a historic shift in the automobile industry to more environmentally friendly electric vehicles.

"Remember he wants to take your jobs away and give them to China," Trump wrote on his social media site Truth Social, in a message that was largely in written in block capitals.

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

However, Biden says his push on 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.

His Michigan trip carries a political risk, however, 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's Jean-Pierre deflected a barrage of questions about whether Biden was taking sides in the dispute, saying the president wanted a "win-win" agreement.

"What we're saying is we're not going to get into this when it comes to negotiation," she added.

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