US President Joe Biden won the coveted election endorsement of America's biggest car workers union on Wednesday, in a major boost to his battle with Donald Trump for the crucial blue-collar vote.

Democrat Biden has relentlessly courted the union vote and appeared on a United Auto Workers (UAW) picket line last year during a strike against the big three US car giants.

"Our endorsements must be earned. Joe Biden has earned it," UAW chief Shawn Fain told a cheering crowd at a conference in Washington, before Biden took the stage in a black union baseball cap.

Union members booed when Trump's name was mentioned and shouted "Joe!" when Fain asked whom they wanted to be president after the November election.

"I have your back and you have mine," Biden told the crowd.

But his speech was briefly disrupted for the second day in a row when a small number of protesters against Israel's war in Gaza started chanting, before being dragged from the room by security guards.

The UAW called for a ceasefire in Gaza in December, becoming the largest US union to do so – and putting it at odds with Biden who has firmly backed Israel since the October 7 attacks by Hamas.

The union's endorsement came at a significant moment as Biden's campaign focuses on a rematch with Trump, after the former president all but secured the Republican nomination in the New Hampshire primary on Tuesday.

Biden and Trump made duelling visits to Michigan, a historic car-producing state that is set to be a key battleground in the election, in the space of a few frantic days in September. (AFP)