Originally Published: 03 MAY 23 15:38 ET

Updated: 03 MAY 23 16:14 ET

By Kevin Liptak, CNN

(CNN) -- The powerful United Auto Workers is holding off on endorsing President Joe Biden's reelection bid, citing concerns over his policies that would encourage a transition to electric vehicles, according to a memo from the union.

Biden, who has termed himself the most "pro-union" president in history, has already received the backing of some unions, including Service Employees International Union and the International Brotherhood of Electrical Workers. His first stop after announcing his reelection campaign last week was to a union conference in Washington.

But the Detroit-based UAW is stopping short of backing the president, whom it endorsed in 2020, as Biden works to enact policies meant to combat climate change, including encouraging the manufacture of electric vehicles.

"The United Auto Workers is not yet making an endorsement," the group's president Shawn Fain wrote in a memo to members.

The UAW has more than 400,000 members, and Biden has touted its support in the past. Last year he called American autoworkers "the most skilled autoworkers in the world." The group's membership is mostly concentrated in Michigan, a presidential election battleground.

Leaders from the group met in Washington last week with top Biden administration officials to voice their concerns over the electronic vehicle policies.

Biden has endorsed policies meant to significantly transition the nation's autos to electric, including rules from the EPA that would ensure two-thirds of new cars sold in the United States are electric by 2032.

The transition is meant to curb emissions that are a leading cause of climate change. But autoworkers are expressing concern the transition could leave them out.

Electronic vehicles are easier to make in large part because they have fewer components. Internal combustion vehicles and their powertrains -- the mechanics that convert gasoline to energy and propel the vehicles -- have far more individual components to assemble. Electronic vehicles are powered by batteries and use energy more efficiently, according to a Department of Energy analysis.

"The federal government is pouring billions into the electric vehicle transition, with no strings attached and no commitment to workers," Fain wrote in the memo. "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 has traditionally supported Democrats, and Fain writes in his memo that "Another Donald Trump presidency would be a disaster."

Still, the memo says UAW members "need to see an alternative that delivers real results."

"We need to get our members organized behind a pro-worker, pro-climate, and pro-democracy political program that can deliver for the working class," the memo reads.

Biden has long relied on the political support of unions, and earlier Wednesday his campaign released a television ad leaning into his economic record.

The 60-second ad titled "Backbone" strikes a populist tone, mixing audio of the president speaking about "investing in places and people that have been forgotten" and a narrator ticking through the administration's work to boost infrastructure and manufacturing in the country.

"Joe Biden's building an economy that leaves no city, no town, no American behind," the narrator says.

This ad is part of the seven-figure ad campaign in six battleground states announced last week. The campaign has also expanded their buy into two new states -- Florida and North Carolina -- which Biden lost during the 2020 campaign.

This story has been updated with additional developments.