The head of the United Auto Workers union is publicly denouncing former President Donald Trump ahead of his visit next week to Detroit as part of a plea to current and former union members.

"Every fiber of our union is being poured into fighting the billionaire class and an economy that enriches people like Donald Trump at the expense of workers," UAW President Shawn Fain said in an emailed statement. "We can't keep electing billionaires and millionaires that don't have any understanding what it is like to live paycheck to paycheck and struggle to get by and expecting them to solve the problems of the working class."

In his Detroit speech, Trump may sense a political opportunity from the autoworkers strike, attempting to gain support from working-class Americans who are increasingly disappointed in the direction of the US economy. Trump's rally would take place concurrently with the second Republican presidential primary debate in California, which Trump plans to skip. He also skipped the first debate.

Union members, once a reliable Democratic voting block, have gradually gravitated to Republican candidates, according to CNN polls and the AFL-CIO.

But Trump's union rally is no sure bet. He has repeatedly criticized the popular Fain, a surprise winner of the union's recent leadership election after running an aggressive campaign that captured the anti-establishment zeitgeist of the UAW workers. Yet Trump has told union members that the UAW president doesn't hold their best interests at heart.

"The auto workers are being sold down the river by their leadership, and their leadership should endorse Trump," he told NBC's "Meet the Press" this week.

Trump said that the union should completely reject the Biden administration's push toward electric vehicles as "non-negotiable."

In reality, there's little the union can do to prevent the transition to EVs as consumer demand picks up for electric cars and trucks. But Fain has made the transition to EVs one of the primary points of contention with the Big Three automakers. He has called on GM, Ford and Stellantis to agree to a "just transition" to EVs, which the union fears could eliminate union jobs because they require less labor to assemble.

Tough talk for Biden, too

Fain's tough words for Trump aren't a surprise. In a letter sent to union membership in May, Fain said a second Trump presidency would be a "disaster."

But Fain hasn't been overly kind to Biden, either. Although Fain has at times praised Biden, he has also criticized the president for failing to do enough to help the autoworkers in their cause.

The UAW routinely supports Democratic presidential candidates, and the president -- who is fighting low approval ratings ahead of the 2024 presidential election -- could use all the support he can get. But Fain has withheld the UAW's endorsement of Biden.

Fain's lack of deference to the White House has added pressure on the notoriously pro-union president. Biden, meanwhile, called on major American automakers to improve their offer to workers, saying he "respected" workers' right to strike.

But some Democratic politicians are urging Biden to do more. California Rep. Ro Khanna on Monday told CNN's Vanessa Yurkevich that Biden and other Democrats should join the pickets.

"I'd love to see the president out here," he added, arguing the Democratic Party needs to demonstrate it's "the party of the working class."