The United Automobile Workers, a major US union representing more than 400,000 people, has endorsed the re-election bid of President Joe Biden.
It comes after the Democrat, 81, joined UAW workers on a picket line last year, in a first for a sitting president.
Mr Biden is expected to face off in the November general election against Donald Trump, who polls more strongly among blue-collar voters.
The UAW endorsement may boost his efforts to eat into that advantage.
Though the UAW backed Mr Biden ahead of the 2020 election, its new leadership declined last year to join other high-profile unions in endorsing his re-election. President Shawn Fain had said a 2024 endorsement should "be earned, not freely given".
But Mr Fain said Wednesday that members, mostly workers in car and truck factories, face a choice between voting for "someone who stands up with us and supports our cause" or for "someone who will divide us and fight us every step of the way".
"If our endorsements have to be earned, Joe Biden has earned it," he said at the close of the UAW's three-day national conference in Washington DC.
Mr Biden - who often refers to himself as the most pro-union US leader - donned a UAW ballcap and said "it's great to be home" as he accepted the endorsement.
"I'm honoured to have your back and you have my back. That's a deal," he told a sea of rank-and-file union members in red polo shirts.
In September, the president - with bullhorn in hand - joined union members on strike from a General Motors parts warehouse in Detroit, Michigan.
The unprecedented show of support came as the UAW took on the big three US automakers - GM, Ford and Stellantis - all at once in contract negotiations, by going on individual strikes with little notice and on a rolling basis.
Mr Trump, who held an event a day later at a non-union plant in Michigan, dismissed the incident as "nothing more than a PR stunt".
But by November, union leaders had brokered a historic new contract that included record wage hikes and cost-of-living adjustments.
"You built these companies, you sacrificed to save them in the worst of times and you deserve to benefit when these companies thrive," Mr Biden said on Wednesday.
"The days of working people being dealt out of the deal are over in this country."
Mr Fain praised the president for his role in the negotiations, saying he had "heard the call" and "joined us in solidarity" in addition to vocally supporting the right to strike and creating new opportunities to organise.
By contrast, the previous president had "trashed our union" and taken actions to "screw the American middle class", he alleged.
The UAW endorsement may motivate its members to vote for Mr Biden, who has been plagued by low approval ratings and is struggling to build enthusiasm for his re-election bid.
This is particularly true for those workers concentrated in the auto-making battleground state of Michigan, where Mr Biden needs to cut into his opponent's lead with working-class voters.
Though he defeated Mr Trump in the state in 2020, Mr Biden is facing a revolt this year from its sizeable Arab-American electorate as well as many young voters over his position on the Israel-Gaza conflict.
Offering a preview of those electability concerns, protesters demanding a ceasefire in Gaza interrupted Mr Biden's remarks, in what is becoming a common occurrence at his public events.
