President Biden received the endorsement from the United Auto Workers Wednesday, as the president of the union, Shawn Fain, rejected President Trump and raised the prospect of a general strike in 2028 to reverse the downward trajectory of Big Labor.

Mr. Biden, who has often referred to himself as the “most pro-union president in history,” received the endorsement from the UAW months after appearing on a union picket line during their strike in the fall. The union had been withholding its endorsement in part due to its concerns about the administration’s push for more electrical vehicles.

“Look, I kept my commitment to be the most pro-union president ever,” Mr. Biden said. “I’m proud you have my back. Let me just say I’m honored to have your back, and you have mine.”

Mr. Trump had attempted to court the union members. However, in remarks at the union’s conference Mr. Fain took the opportunity to express his disapproval for the GOP frontrunner, saying “Donald Trump is a billionaire and that’s who he represents.”

“If Donald Trump ever worked in an auto plant, he wouldn’t be a UAW member — he’d be a company man trying to squeeze the American worker,” Mr. Fain said, according to the union’s social media postings.

Mr. Fain went on to call Mr. Trump a “scab,” saying that he had the opportunity to support the union in their 2019 strike but didn’t do so, “He said nothing, he did nothing — not a damn thing, because he doesn’t care about the American worker.” 

In recent months, Mr. Fain has emerged as an outspoken leader in the American labor movement and at the beginning of the union’s conference at Washington D.C., he signaled ambitions of inspiring labor action beyond the UAW.

On Monday, Mr. Fain said that America’s unions needed to “pay for our sins of the past,” referring to President Reagan’s move to fire striking federal air traffic control employees in 1981.

The employees were members of a union, the Professional Air Traffic Controllers Organization. “Back in 1980,” Mr. Fain said, “when Reagan at the time fired Patco workers, everybody in this country should have stood up and walked the hell out.”

“We missed the opportunity then, but we’re not going to miss it in 2028. That’s the plan,” Mr. Fain said. “We want a general strike. We want everybody walking out just like they do in other countries.”

Despite Mr. Fain’s ambitions of a more coordinated labor movement, union membership in America has fallen to an all-time low. 

The Bureau of Labor Statistics reported Tuesday that only 10 percent of hourly and salaried employees in America were members of unions in 2023, down from 10.1 percent in 2022. 

The level of unionization in the private sector is even lower, with only six percent of private sector employees being in a union. In the public sector, the unionization rate was 32.5 percent.

While Mr. Biden won the endorsement of the union, the votes of the union membership are still up for grabs in 2024. Mr. Trump had attempted to influence members to pressure leadership into endorsing him, to no avail. 

The UAW, however, has some 400,000 members — many of whom are located in Michigan and other competitive states — and Mr. Trump will likely attempt to appeal directly to the auto manufacturing employees, as he did last year, when he visited a non-union plant during the strike.

Since 2017, the proportion of union members who align themselves with the Democratic Party has increased to 51 percent from 40 percent, according to a recent Morning Consult survey. Simultaneously, alignment with the Republican Party fell to 23 percent from 30 percent.