Joe Biden, the self-proclaimed "most pro-union" American president, on Wednesday secured the United Auto Workers Union's endorsement as he ramps up his reelection bid, after the group had withheld backing him for months.
But the Michigan-based union's support -- which includes more than 400,000 active members and millions in political spending -- was not earned without courting by the president as his campaign says he's going "full steam ahead" toward a likely rematch against Donald Trump.
"I'm proud you have my back," Biden said at the UAW's conference in Washington, D.C., on Wednesday after union President Shawn Fain officially endorsed him. "Let me just say, I'm honored to have your back and you have mine -- that's the deal."
The UAW, which backed Democrats in the two previous presidential elections, was holding out in part because of some concerns among members about what Biden's electric vehicle policies could mean for their jobs.
Fain on Wednesday appeared to try to quell his members' fears.
"Advances in technology don't have to leave the plants closing or leave the remaining workers working harder than before," Fain said. "We should be the masters of technology, not let it master us and force us to work even more for less."
Later Wednesday, Biden upheld an administration waiver to allow EV chargers to be made using foreign components if final assembly occurred in the U.S.
Jonathan Hanson, a lecturer at the Gerald R. Ford School of Public Policy at the University of Michigan, told ABC News that having "vocal union leadership" embracing Biden's EV push can help prevent swayable members from again voting for Trump.
"To the extent that the union leadership is able to articulate a message that they're on board with this transition, because they've gotten assurances from the president, that could serve to weaken the power of those attacks that are going to be coming from Trump," Hanson said.
Biden has taken other big, even historic, steps to support union workers -- historically a core constituency for Democrats that had switched to Trump in 2016, helping elect him to the White House.
A person familiar with Biden's thinking told ABC News, of his work to win over the UAW, that the president always expects to have to earn such endorsements.
Biden in September became the first modern president to join a picket line when he traveled to Detroit to publicly back UAW members during their strike against America's three largest car manufacturers.
"Folks, stick with it because you deserve the significant raise you need and other benefits," Biden, donning a UAW baseball cap and speaking through a bullhorn, told strikers at the time.
He later traveled to Illinois to deliver remarks to an audience of UAW workers and applaud them for striking and securing a deal.
It was never likely that the UAW, which had backed Biden in 2020, would flip and support Trump. Fain, the union president, has been vocal in his stinging criticism of the former president.
In his speech on Wednesday, Fain called Trump a "scab" and said he did "not a damn thing" during the UAW strike against General Motors in 2019, when he was president, "because he doesn't care about the American worker."
Trump's campaign did not respond to a request for comment on Fain's criticism, but Trump has previously attacked Biden's union record as "nonsense" and said he is focused on protecting American auto labor, including from EV policies.
Fain, despite his clear distaste for Trump, had said last year that the group's endorsement must be earned -- raising questions if Biden would secure it.
"I really think they were flexing their muscles," Hanson, the public policy expert, told ABC News of the UAW's delay. "They knew that Biden needed this endorsement. They know the race is close."
Beyond being based in a critical swing state that helped decide the 2016 and 2020 elections, UAW has expansive resources.
The union and its local chapters have joined get-out-the-vote efforts in the past -- such as phone banking and door knocking -- and, according to tracking by OpenSecrets, their affiliated political group spent more than $14 million in campaign-related expenses in 2020.
Fain told his members on Wednesday: "We've said we'd stand with whoever stood with us in our fight, not because somebody was nice to us, and we want to be nice to them, but because we need to know who's gonna put up and who's gonna shut up."
Biden has leaned heavily on union support during his long career, including in his run in 2020, when he clawed back some Rust Belt blue-collar support that Democrats lost in 2016.
Exit polling in 2016 showed former Secretary of State Hillary Clinton faltered with union workers in Ohio and Michigan. Trump beat her in both.
In 2020, Biden narrowly won back Michigan and two other Rust Belt states, Pennsylvania and Wisconsin, that Trump won four years earlier -- which had made up Democrats' so-called "blue wall" in the Midwest.
The Biden-Trump showdown for union workers came to a head when Biden joined the picket line last September.
The next day, across town, Trump spoke about the strike at a rally.
The Trump team slammed Biden's visit as a "PR stunt," while the Biden campaign hit Trump for holding his event at a non-union shop. (Many of the UAW attendees at Trump's event were not on strike.)
Fain, in his remarks on Wednesday, was critical of Trump's Detroit-area stop.
"He went to a non-union plant, invited by the boss, and trashed our union," Fain said of Trump. "That's right. And here is what Joe Biden did during our stand-up strike: He heard the call, and he stood up, and he showed up."
"Perhaps even more important for Biden was not just the support from the union, but the rather visceral attack that the union president, Shawn Fain, made on Donald Trump," Hanson said.
As Biden accepted the UAW endorsement on Wednesday, he also hit Trump for "attacking unions" and "leaving too many Americans behind" and said Americans "lost their sense of pride" during the Trump administration because jobs were going overseas.
That tone was no accident: The Biden campaign told reporters on Wednesday that they are putting their full focus and energy into challenging Trump, declaring the start of their general election campaigning even as Trump still faces challengers in his race for the GOP nomination.
"It's all hands on deck now," Quentin Fulks, Biden's principal deputy campaign manager, said. "We are full steam ahead heading into the general election."
In further signs the campaign is positioning itself for the general, Biden is moving two of his top advisers, Mike Donilon and Jen O'Malley Dillon, who managed his 2020 campaign, from the White House to his reelection team.
"Put simply, Trump's party is divided and now he's about to face the only politician who has ever beaten him and who did so with more votes than any presidential candidate in history: President Joe Biden," Fulks told reporters.
He argued that while the primary race shows Trump continues to be dominant with the GOP base, there are signs he is "struggling to make himself palatable" to some key constituencies, such as independents and college grads, who will help decide the race in November.
Asked by reporters about head-to-head polling that a Biden-Trump matchup would be a tight race, with Trump leading in some surveys, campaign co-chair Cedric Richmond tried to brush off the numbers while stressing, "We're going to run like we're behind," before adding, "Are we going to win? Absolutely."
But Biden has faced persistently low approval numbers for months, with many Americans sour on his handling of issues like the economy and immigration.
He has drawn particular ire from some young Americans and Arab Americans over his handling of the Israel-Hamas War, as evidenced by protests on Tuesday at a campaign rally in Virginia.
"I think what you saw [Tuesday] was a president who understands and respects Americans fundamental First Amendment rights to peacefully protest," communications director Michael Tyler told ABC News Senior White House Correspondent Selina Wang on Wednesday. "I think that stands in stark contrast to Donald Trump and the Republicans who don't seem to understand the same thing, who only want to use these situations to fan the flames and further divide people."