Detroit, Michigan - Kamala Harris and Donald Trump invited star power to the campaign trail Saturday, as they took shots at each other's endurance and urged early voting in battleground states key to the ever-tightening presidential race.

At rallies in Detroit and Atlanta Harris brought out pop stars Lizzo and Usher respectively to warm up her crowds, while painting her rival Trump as exhausted and unhinged.

The Republican running for a second go in the White House countered those accusations with a marathon speech in Pennsylvania, as billionaire Elon Musk campaigned for him elsewhere in the state.

Both candidates are fighting to seal up voters' support in a race that polls suggest is effectively tied with fewer than three weeks to Election Day.

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Harris told voters in Detroit that her opponent's platform is "self-consuming" while repeating vows to invest in the working and middle classes.

"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," said Harris.

Later in Atlanta, Harris, who turns 60 on Sunday, accused 78-year-old Trump of "ducking debates and canceling interviews because of exhaustion."

"When he does answer a question or speak at a rally – have you noticed he tends to go off script and ramble, and generally, for the life of him, cannot finish a thought?" she said.

"He's called it the weave. But we here, we will call it nonsense."

Trump began his more than 90-minute rally with a lengthy monologue on the late golfer Arnold Palmer, for whom the regional airport in Latrobe, where the Republican appeared, is named.

He went so far as to praise Palmer's genitalia.

"When he took the showers with other pros, they came out of there, they said, 'Oh my God. That's unbelievable,'" Trump said with a laugh. "I had to say it."

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He then launched into his routine, meandering speech that includes attacking migrants, personally denigrating Harris, and repeating false claims about the 2020 election.

Tump's rally also included a number of guests and screenings of his filmed campaign ads.

Shortly after recalling his own expensive education at the private Ivy League University of Pennsylvania, Trump vied to appeal to working-class voters by bringing a parade of steel workers in hard hats onstage.

He also underscored the importance of the state's electoral college delegates to the overall election: "If we win Pennsylvania, we win the whole damn thing."

At rally in Las Vegas, former President Barack Obama took aim at Trump, comparing him to a grandfather whose bizarre behavior would spark worry after his rambling speeches and a strange dance party.

"So you would be worried if your grandpa started acting like this. You would... right? You'd like, call up your brother... be like, have you seen grandpa lately? What we gonna do?"

"But this is coming from somebody who wants unchecked power, wants the most powerful office on Earth, with the nuclear codes and all that," said Obama.

Earlier in the day, pop star Lizzo noted that "whether you're a Democrat or Republican or neither, you deserve a president who listens when you speak."

"You deserve a president who respects when you protest. You deserve a president who understands that their job is to be a public servant," she said before emphasizing that Harris offers just that.

The vice president, meanwhile, has struggled to win over Muslim and Arab-American voters in Michigan and elsewhere, who have voiced outrage over her refusal to shift her policy toward Israel amid the relentless attacks on Palestine and Lebanon.

Lizzo – who sported a suffragette-white pantsuit as she addressed the crowd in Motor City – drew cheers when urging listeners that America was more than ready for its first woman president, dropping a reference to her own hit song: "It's about damn time!"

One of Atlanta's major stars, Usher, told voters there that "I'm counting on you" to get Harris' "campaign across the finish line" in Georgia.

Both candidates are spending their final campaign days in pivotal battleground states where early voting is already underway.

Musk, who endorsed Trump in July, is one of the Biden administration's fiercest critics and has emerged as a loud voice in US politics since taking over Twitter, now known as X.

The Tesla and SpaceX CEO has taken an increasingly visible role in Trump's campaign and has donated almost $75 million to his political organization America PAC.

Speaking in Harrisburg, Pennsylvania, Musk announced he would start randomly distributing cash awards – $1 million each day until the November 5 vote – to a registered voter in the state who signed his organization's petition.