CHICAGO — United Auto Workers President Shawn Fain on Monday said Vice President Kamala Harris will fight for working-class voters and the union movement.

“For us in the labor union, it’s real simple, Kamala Harris is one of us. She is a fighter for the middle class and Donald Trump is a scab,” Fain said from the stage at the Democratic National Convention

Mr. Fain spoke while wearing a bright red shirt reading, “Trump is a scab.”

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Mr. Fain was among the first major union leaders to endorse Ms. Harris for president, declaring his support for her just days after she replaced President Biden on the Democratic ticket.

The Detroit-based union launched an operation to mobilize its members to vote for Harris, pledged $1.5 million to the Democratic National Committee and filed a federal labor complaint against Mr. Trump.

He blasted Mr. Trump and his running mate, Sen. J.D. Vance of Ohio, as “lapdogs for the billionaire class who only serve themselves.”

Organized labor leaders have quickly rallied around Ms. Harris. She has picked up endorsements from the AFL-CIO and most other unions.

But support for Ms. Harris among rank-and-file union workers is soft. At the time Mr. Biden dropped out of the race, he was tied with Mr. Trump, the GOP nominee, at 47% each among union households.

In 2016, Mr. Trump’s support of blue-collar workers helped him flip Michigan, Pennsylvania and Wisconsin into the GOP column. In 2020, those workers returned to the Democratic Party as Mr. Biden won 56% of union households, according to AP VoteCast.

Mr. Fain blasted Mr. Trump for a 2018 promise to bring back a shuttered General Motors plant in Youngstown, Ohio, but the jobs never returned.

“Trump did nothing. Trump told the workers that he would bring all the auto jobs back and he did nothing,” Mr. Fain said.

Mr. Trump has denounced Mr. Fain as a failed leader and called him “a puppet for the Democratic Party” who sold out rank-and-file union members.