One of the largest labor unions in the U.S. is withholding its reelection endorsement of President Biden over concerns about the transition to electric vehicles in the automobile industry, according to a report.

An internal letter at the United Auto Workers (UAW) obtained by CNBC shows the union's president promising to push for answers on how investments in electric vehicles may impact his members before he makes endorsements. 

"The federal government is pouring billions into the electric vehicle transition, with no strings attached and no commitment to workers," UAW President Shawn Fain wrote in a letter to his employees. "The EV transition is at serious risk of becoming a race to the bottom. We want to see national leadership have our back on this before we make any commitments."

The UAW primarily donates to Democratic candidates at the federal level and previously endorsed Biden for president in 2020. Fain wrote in the letter that "another Donald Trump presidency would be a disaster." The union has more than 400,000 active members and is based in Detroit.

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The UAW has expressed concerns over how investments to transition automobile manufacturing to electric vehicles may impact job losses from his members. Fain noted recent automobile plant closures and unjust pay rates, which he tied to an increased focus on electric vehicles.

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"Right now, we’re focused on making sure the EV transition does right by our members, our families, and our communities," Fain wrote in the letter. "We’ll be ready to talk politics once we secure a future for this industry and the workers who make it run."

Biden often boasts he is the "most pro-union president in history"