Trump responded by lashing back about the energy he's shown on the campaign trail.
Harris, who turns 60 on Sunday, pressed the case to raise doubts about the 78-year-old Trump. Age had been an issue when President Joe Biden, 81, was still in the race, but had faded after he dropped his election bid.
Harris said news reports that former president Trump was skipping interviews because he was tired and had passed on the chance of a second debate with her raised questions about his fitness for office.
Kamala Harris and Donald Trump are battling fiercely to win Michigan at the November election. (AP PHOTO)
"It should be a concern. If he can't handle the rigours of the campaign trail, is he fit to do the job?" she told reporters before a rally in Grand Rapids. "That's a legitimate question."
Trump has skipped some appearances, but his campaign has not provided reasons.
Trump, talking to reporters as he arrived in Detroit, rejected such talk.
"I've gone 48 days now without a rest," he said.
"I'm not even tired. I'm really exhilarated. You know why? We're killing her in the polls, because the American people don't want her."
Polls in the election's most competitive states are effectively tied.
In a Fox & Friends interview, Trump also griped about negative television ads on Fox about him and said he would ask Rupert Murdoch, the founder of Fox News parent company News Corp, to ensure such ads are not broadcast until election day on November 5.
"I'm going to say, 'Rupert, please do it this way and then we're going to have a victory, cause everyone wants that'," Trump said.
Trump visited a campaign office in Hamtramck, where he heard praise from the Detroit suburb's first Muslim mayor, Amer Ghalib.
The former president was seeking support from Arab Americans in Michigan disenchanted with Democrats, Vice President Harris and Biden over US support for Israel in the Gaza conflict.
"We all ultimately want one thing. We want peace in the Middle East. We're going to get peace in the Middle East. It's going to happen very fast. It can happen with the right leadership in Washington," Trump said, without elaborating.
Donald Trump returned to Detroit eight days after making disparaging remarks about the city. (AP PHOTO)
In the evening, Trump returned to Detroit, Michigan's largest city, for a rally after saying on October 10 that the rest of the US would turn into Detroit if Harris won.
There, Trump's microphone stopped working and the former president roamed around the stage for some 20 minutes.
"I won't pay the bill for this stupid company that rented us this crap," Trump said after the audio started working again. "This is the worst mic I've ever had in my life."
The dead-mic incident took place days after Trump stopped talking and swayed and bopped to his musical playlist at a Pennsylvania town hall event after two people in the audience fell ill.
The state of Michigan has about 8.4 million voters and would bring the winner 15 Electoral College votes out of the 270 needed to win, which could be a decisive number.
Harris and Trump are battling fiercely for the state's Arab American, senior, union and working-class voters.
On Thursday, Harris said Trump was "gaslighting" the American public about the deadly attack by his loyalists on the US Capitol on January 6, 2021. Trump recently called the assault a "day of love".
Public and internal campaign polls show razor-thin margins for either Harris or Trump in Michigan and other battleground states. That's worrying Democrats.
Trump won Michigan by 11,000 votes in 2016. In 2020, Biden beat Trump by 155,000 votes.
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