Republican Donald Trump escalated his verbal attacks on Democrat Kamala Harris on Saturday as the vice president turned to celebrities to generate enthusiasm for her campaign, with both candidates for U.S. president courting early voters.

Harris and Trump are essentially tied in the most competitive states and many Americans are voting early by mail or in person, with just 17 days until the Nov. 5 election.

On Saturday, Harris rallied supporters at a get-out-the-vote event in Detroit, Michigan, for the city's first day of early voting, alongside Detroit-born rapper Lizzo, who said voting was crucial in a state where thin margins can determine elections.

In Atlanta later, Harris appeared with pop singer Usher.

"This is the swing state of all swing states, so every single last vote here counts," Lizzo said. She rejected the argument by some that America was not ready for a woman president, saying, "It's about damn time," riffing on one of her biggest hits.

Votes in a handful of swing states, where candidates from both parties have strong support and electoral results have been close in past cycles, are likely to decide the election.

In Latrobe, Pennsylvania, Trump escalated his personal attacks on Harris, a practice some of his advisers fear could alienate swing voters.

Trump said Harris is further to the left than Democratic Senators Elizabeth Warren and Bernie Sanders.

“You have to tell Kamala Harris that you’ve had enough, that you just can’t take it anymore. We can’t stand you, you’re a shit vice president.”

As the crowd applauded, Trump added: “Kamala, you’re fired, get the hell out of here, you're fired.”

Trump had billed his Latrobe event as the start of his final argument to voters but quickly went off script with a long story about hometown hero Arnold Palmer that included an off-colour comment about the genitalia of the late golf legend.

"He took showers with the other pros, they came out of there and said 'Oh my God, that's unbelievable,'" he said.

The Harris campaign quickly seized on the comments.

"Trump's Pennsylvania closing argument is literal junk. In a Pennsylvania rally speech, his campaign team billed as 'the beginning of his closing argument in the final stretch,' Donald Trump focused on the issue most important to voters in this election: a deceased golfer’s … anatomy," the campaign said in a statement.

In both Detroit and Atlanta, Harris urged her supporters to put in an all-out effort to win.

"On Election Day, we don't want to have any regrets about what we could have done these next 17 days," she told about 300 voters who planned to march together to an early-voting site.

She said early voting had already set records in Georgia and North Carolina, and challenged Detroit - a city known for producing musical recordings - to follow suit.

Harris' next stop was Atlanta, Georgia, for a rally with another music icon, Usher, who took a break from a three-date concert tour in the southern city to urge voters to go door-to-door and enlist their neighbors and friends to vote.

"Every call matters. Every conversation matters. All of it makes a difference in closing this race ... Everything that we do in the next 17 days will affect the future of our children, of our grandchildren, of the people who we love the most," he said.

Harris, speaking before some 11,000 people at a music venue in Atlanta, hammered Trump for a second straight day for cancelling events and for avoiding another presidential debate because of what she called "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 has called it 'the weave.' But I think we here will call it nonsense."

Harris will need strong results in the majority non-white cities of Detroit and Atlanta and their surrounding suburbs to repeat President Joe Biden's 2020 wins in Michigan and Georgia.

Trump is seeking to take advantage of what he felt was an improved position for him in opinion polls that show a deadlocked race. Some voters already have mail-in ballots in the state, which is the biggest prize on Election Day among battleground states and could tip the 2024 race.

Early voting also starts on Saturday in Nevada, where former President Barack Obama plans to campaign for Harris in Las Vegas.

Both 2024 candidates spent Friday in closely contested Michigan, trading jabs about their fitness for office. Trump, 78, dismissed accusations from Harris that he was exhausted by the pace of the campaign's closing days. Harris turns 60 on Sunday.

Trump said he had not cancelled any events. But an Oct. 22 National Rifle Association of America event in Savannah, Georgia, that he had planned to attend, was canceled, according to organizers.