Vice President Kamala Harris let her T-shirt do the talking in Detroit on Saturday.

The black shirt — which she wore under a gray blazer as she addressed several hundred campaign volunteers in a gym at Western International High School — bore the words “Detroit vs. Everybody.” The attire was a clear response to former President Donald J. Trump, who last week disparaged what is one of the nation’s largest majority-Black cities, portraying Detroit as a decaying harbinger of America’s future under Ms. Harris.

In brief remarks to the crowd on the inaugural day of early voting in the city, Ms. Harris urged her supporters to reject Mr. Trump’s division and insults.

“We stand for the idea that the true measure of the strength of a leader is not based on who you beat down, it’s on who you lift up,” she said, saying that her campaign was seeking the kind of “grit” and “excellence” possessed by “the people of Detroit.”

“He spends full time talking about himself and mythical characters, not talking about the working people, not talking about you, not talking about lifting you up,” Ms. Harris added.

Mr. Trump had attacked Detroit while giving remarks at an economic forum in the city on Oct. 10, earning widespread scorn from Democrats and offering fodder for a Harris campaign ad. “Our whole country will end up being like Detroit if she’s your president,” he had warned of Ms. Harris.

Black voters, especially Black men, are supporting Ms. Harris with less enthusiasm than they had for the Democratic nominee in previous elections, and Mr. Trump has tried to take advantage. The Harris campaign has lately ramped up outreach efforts to Black voters, including by releasing an economic policy agenda designed for Black men. Turnout in Detroit could help decide the race in Michigan, one of the nation’s top battleground states, where polls show an even contest.

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