Washington DC - President Joe Biden won the coveted election endorsement of America's biggest car workers union 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 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.

Fain said Trump wanted to "screw the American working class."

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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. Around a dozen pro-Palestinian supporters chanting slogans accusing Biden of genocide also rallied outside the conference, a day after protesters repeatedly disrupted Biden's speech on abortion.

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.

Rank-and-file members of the union rallying for Palestine took to social media to confirm that they did not consent to the endorsement, writing on X, "We were informed of this endorsement after our democratically-elected UAW leaders voted in secret to endorse."

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 dueling 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.

Winning over working-class voters was key to right-wing populist Trump's shock election win in 2016 and to Biden's in 2020 – with little doubt that in both camps, it will be again.

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A megaphone-wielding Biden became the first sitting US president in history to visit a picket line when he appeared along the UAW's Fain in Michigan to back striking workers.

A long-term union supporter, the Democrat repeatedly pushed car makers Ford, General Motors, and Stellantis to offer unions a fair deal and end the strike.

The president made a fresh attempt to rev up UAW support in November, almost exactly a year before the election when he donned a union T-shirt and addressed workers in Illinois who had won the reopening of a factory.

He lashed out then at tycoon and former reality TV star Trump for only visiting non-union workers during his trip to Michigan last year.

On his own visit, Trump charged that Biden's push for electric vehicles amounted to "a government assassination of your jobs and of your industry."