Gov. Tim Walz of Minnesota took the stage in a Detroit suburb on Friday to offer a sharp rebuttal to former President Donald J. Trump, who had positioned himself as a savior of the auto industry at an appearance in Detroit a day earlier.

Speaking to about 100 people inside a community college’s fabrication shop in Warren, Mich., Mr. Walz argued that the Trump economic agenda would be harmful to blue-collar workers and manufacturing in the state, a key battleground for the presidential race between Vice President Kamala Harris, the Democratic nominee and his running mate, and Mr. Trump.

“Trump has spent his life talking a big game, but he has been an absolute disaster for working people,” Mr. Walz said. “One of the biggest losers of manufacturing jobs of any American president in history.”

He blamed Mr. Trump for the loss of about 280,000 jobs in Michigan during the pandemic, suggesting the former president’s “disastrous mismanagement” of Covid, his trade wars and the federal contracts he gave to businesses that off-shored jobs were to blame. And he said in a mocking tone that Mr. Trump had kept his word when he promised, in a 2016 speech in the same town where Mr. Walz was speaking, that autoworkers wouldn’t “lose one plant” if he were elected.

“Technically, it wasn’t a lie, because he lost six of them,” Mr. Walz said. A number of auto plants, including several owned by General Motors, closed during Mr. Trump’s presidency.

The Trump campaign argued in a statement that Mr. Trump had created jobs for auto workers before the pandemic and negotiated the United States-Mexico-Canada Agreement, a trade deal that included incentives to make cars in North America. The campaign also said he would help the manufacturing industry if re-elected by imposing tariffs against other countries.