Former President Donald J. Trump was roughly an hour and a half into a nearly two-hour speech to the Detroit Economic Club on Thursday afternoon before he got to his main new policy proposal: a call to make car loan interest fully tax deductible.

The proposal, which came late during a circuitous speech to business leaders, merged two of Mr. Trump’s favored efforts to win voters: targeted tax cuts aimed at key voting blocs nationwide and promises to revitalize the auto industry in Michigan, a critical battleground state. Even before this latest tax cut proposal, the Committee for a Responsible Federal Budget estimated that Mr. Trump’s agenda could add as much as $15 trillion to the nation’s debt over a decade.

Mr. Trump claimed his plan would “stimulate massive domestic auto production and make car ownership dramatically more affordable” for families.

But before he got to his new proposal, Mr. Trump often rambled, reviving his false claims about the 2020 election, mocking President Biden’s 2020 campaign crowds, praising Elon Musk’s rockets. At one point, as he criticized Vice President Kamala Harris, Mr. Trump did something that politicians rarely do: He took a pointed dig at the city that was hosting him.

“Our whole country will end up being like Detroit if she’s your president,” he said. “You’re going to have a mess on your hands.”

Gov. Gretchen Whitmer of Michigan, a Democrat, blasted Mr. Trump’s comments in a social media post, saying that “you better believe Detroiters won’t forget this in November.”