Waterford Township, Michigan - Kamala Harris questioned Donald Trump's fitness for office Friday as the oldest major-party White House candidate in history faced speculation that he is "exhausted" after backing out of a spate of interviews.

While he has been appearing on friendly TV networks, the 78-year-old Republican has canceled sitdowns with media outlets including NBC, CNBC and CBS. He has also refused a second debate with Harris, after being soundly bested in the first.

Politico reported that a Trump aide had told producers at a website negotiating an interview that the ex-president was "exhausted" and refusing some appearances – a claim described by his campaign as "detached from reality."

But Harris, who turns 60 this weekend, hammered Trump over his health and resilience.

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"If you are exhausted on the campaign trail, it raises real questions about whether you are fit for the toughest job in the world," Harris told supporters during a day of multiple stops across the swing state of Michigan.

The former president has hardly been idle, pursuing a busy schedule of appearances with new and traditional media, but most have been on outlets where he is rarely challenged.

Trump reacted angrily to Harris' jab, telling reporters that he had canceled nothing and calling his Democratic opponent a "loser" who "doesn't have the energy of a rabbit."

The ex-president also pushed hard at a town hall in Auburn Hills, a suburb of automaking hub Detroit, on his pledge to impose across-the-board levies on US imports.

"You have a lot of words that are damn nice like love. But I think it's more beautiful than love. The word tariff," Trump said.

But Harris, speaking in Oakland County, warned voters: "Donald Trump is making the same empty promises to the people of Michigan that he did before, hoping you will forget how he let you down the last time."

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The vice president has a narrow lead in national polling averages, while numerous October surveys in Michigan show them neck and neck.

Trump has surprised analysts with a program that mixes swing state stops with appearances in regions he has no chance of winning, but where he is guaranteed big crowds. He was in the liberal stronghold of New York for a Catholic charity dinner on Thursday, where he mocked Harris in an occasionally mean-spirited speech that earned gasps for its off-color remarks and profanity.

But he was back on home turf Friday morning for a soft Fox News interview, before heading to Michigan for counter-programming against Harris.

Both candidates are spending their final campaign days in pivotal battleground states where early voting is already underway. With less than three weeks to go, Harris has seen encouraging signs in her push for supporters to vote as soon as possible, as a bulwark against the traditional Republican edge among Election Day voters.

Almost 12 million votes had been cast by Friday evening – around a third of them in the seven swing states expected to decide the election – according to data tracked by the University of Florida Election Lab.

Georgia has been smashing records, while North Carolina reported a first day of voting Thursday that beat 2020, when there was a pandemic-linked surge of early ballots.

Where party breakdowns were available, registered Democrats accounted for roughly half of the total, while Republicans – who have spent much of the Trump era casting aspersions over drop boxes and mailed ballots – were responsible for around a third.

The Democrat has found herself on eggshells as she upholds President Joe Biden's support for Israel, while Muslim and Arab American voters – particularly in Michigan – have voiced outrage over the genocide in Gaza and ongoing US weapons shipments.

The killing of Hamas chief Yahya Sinwar drew optimism from Harris for a Gaza ceasefire, but Israel quickly said his death is not the end of the brutal military assault.

Speaking to journalists ahead of a speech in Detroit, Trump said Sinwar's death had increased the likelihood of a peaceful solution to the war on Gaza – while warning Biden not to try to restrain Israel.