Joe Biden joined striking auto workers on the picket line in Michigan on Tuesday in an 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 an "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. (AFP)