The United Automobile Workers union endorsed President Biden on Wednesday, delivering an influential boost as he faces a battle against former President Donald J. Trump to win the support of labor groups.

Mr. Biden, who calls himself the “most pro-union president in history,” delighted striking U.A.W. workers but angered auto industry executives when he appeared on a picket line with workers last fall. On Wednesday, he appeared in front of a national conference of autoworkers to tell them that he had been proud to do it.

“Let me tell you something I learned a long time ago,” Mr. Biden said. “If I’m going to be in a fight, I want to be in the fight with you, the U.A.W.”

In earlier remarks, Shawn Fain, the U.A.W. president, told the crowd that Mr. Biden had the track record to help working-class people organize for higher wages, better retirement benefits and health care.

“This election is about who will stand up with us and who will stand in our way,” Mr. Fain said after a lengthy speech comparing Mr. Biden’s past pro-union speeches with Mr. Trump’s lack of support and appearances at nonunion facilities. He called Mr. Trump a “scab” — shorthand for someone who undercuts the power of a union, such as by replacing a striking worker.

“If our endorsement must be earned, Joe Biden has earned it,” Mr. Fain said.

The value of the endorsement, which the U.A.W. put off last year over concerns about Mr. Biden’s commitment to promoting union jobs in electric-vehicle manufacturing, may be less about persuading members to back Mr. Biden than in motivating them to vote. The union has estimated that only about 30 percent of its members supported Mr. Trump in 2016. But without the union’s formal backing and investments in turnout, Mr. Biden could suffer a drop-off in members who show up to vote in critical swing states like Michigan.

“Elections aren’t about just picking your best friend for the job or the candidate who makes you feel good,” Mr. Fain said. “Elections are about power.”

With Mr. Trump all but locking up the nomination after his performance in New Hampshire’s primary on Tuesday, officials with the Biden campaign said that the race between the two candidates had all but begun.

While onstage, Mr. Biden focused on highlighting economic bright spots, including a strong job market, lower gas prices and increased consumer optimism. He also compared his track record to his predecessor’s, saying Mr. Trump is “the only president other than Herbert Hoover who lost jobs when he was president.”

Mr. Fain, a vocal critic of Mr. Trump, did not mince words. He recalled the 2008 financial crisis, highlighting Mr. Trump’s anti-union rhetoric then and as a presidential candidate in 2015. Then he recalled Mr. Biden’s comments, as vice president, that the “nation bet on American autoworkers and won.”

At this, attendees yelled out obscenities about Mr. Trump. “Love the energy,” Mr. Fain replied.

After the event, Mr. Fain told reporters that the U.A.W.’s board had unanimously approved the endorsement. He said that Mr. Biden’s economic message was breaking through to rank-and-file members, but added that the union would “have to do better” to combat what he said was misinformation about Mr. Biden’s achievements.

Still, Mr. Fain had made the president work for the endorsement.

Mr. Biden appeared at several U.A.W. events to prove his bona fides with the group’s leadership and rank and file. In September, Mr. Biden grabbed a bullhorn and joined striking autoworkers in Michigan, becoming the first sitting president to join a picket line in an extraordinary show of support for workers demanding better wages. When the contract was won, Mr. Biden wore a red T-shirt and appeared before celebrating workers in Illinois.

Union officials often say Mr. Biden has been more vocal than any president in decades in backing organized labor. He appeared in a video as Amazon workers in Alabama sought to unionize, warning that “there should be no intimidation, no coercion, no threats, no anti-union propaganda.” He also called out Kellogg for its plans to permanently replace striking workers. (The strike was resolved before the company took that step.)

The U.A.W. was early to support Mr. Biden’s green energy policies, but became frustrated by the lack of support for unionized auto-industry jobs in the Inflation Reduction Act, the major climate bill that the president signed in 2022.

Mr. Fain had also expressed frustration that the Biden administration had given Ford a $9 billion government loan to build three electric-vehicle battery plants in Tennessee and Kentucky without any commitment by the company to create high-wage union jobs there.

It takes fewer workers to assemble an electric vehicle than it does to build one with an internal combustion engine. To make up for those lost assembly jobs, the U.A.W. wants to organize the plants that make batteries and other electric vehicle parts that are being built to take advantage of the tax incentives included in Mr. Biden’s climate legislation. They are also pushing to extend union organizing to electric vehicle makers that have long resisted it.

Mr. Biden’s decision to appear on the picket line in Michigan raised the ire of auto industry executives, according to officials in his administration, who said that the president was nonetheless determined to make clear where he stood in the labor conflict.

Seeing an opening with the U.A.W.’s rank and file, if not its leadership, Mr. Trump then made a play for the endorsement, campaigning against Mr. Biden’s “ridiculous Green New Deal crusade.” A day after Mr. Biden joined the U.A.W. picket line, Mr. Trump rallied at a nonunion auto parts factory in Michigan, vying for the support of blue-collar workers.

Mr. Fain had long made it clear his leadership would never endorse the former president, but endorsing Mr. Biden was still politically complicated. In addition to the substantial portion of his membership that is likely to favor Mr. Trump, the U.A.W. also includes a vocal liberal bloc that is skeptical of Mr. Biden. Many of the liberal members are graduate students and university researchers who have been critical of the president over his support for Israel during its war in Gaza. The union itself has called for a cease-fire.

During the event, several protesters waving Palestinian flags were dragged out by security officials as the president kept speaking, the second time in as many days that a Biden campaign event has been interrupted by people protesting Israel’s campaign against Hamas in Gaza. Workers chanted “U.A.W.” to drown out their cries.