At an event at his Florida estate one week before the November 5 election, Trump said Harris would be too dangerous to serve as president, pointing to foreign wars and high levels of immigration that have occurred during her tenure as vice president.
His accusation that Democrats had unfairly forced Biden out of the race recalled Trump's false claims that he had lost the 2020 election due to fraud.
Former President Donald Trump described his Madison Square Garden rally as a "love in". (AP PHOTO)
"They stole the presidency of the United States. You can call it a coup, you can call it whatever. But they stole it," he told supporters.
"The way they took that away from him was not right."
Biden, 81, abandoned his re-election bid in July after weeks of pressure from fellow Democrats who worried he was too old for a rematch with Trump, 78.
Harris is due to hold a rally later in the day at the Ellipse, a park near the White House where on January 6, 2021, Trump urged supporters to "fight like hell" and march to the US Capitol, where congressmen were certifying his loss.
Four people died in the ensuing riot, and one police officer who defended the Capitol died the following day. Trump has said, if he is re-elected, he would pardon the more than 1500 participants who have been charged with crimes.
At Harris' event, which is expected to draw at least 20,000 people, she will call on Americans to "turn the page" on Trump while stressing her plans to lower costs and make the economy work for middle-class Americans, campaign chair Jen O'Malley Dillon told reporters.
Harris' lead over Trump has dwindled in the final weeks of the campaign to a single percentage point, and she now leads 44 per cent to 43 per cent among registered voters, according to a Reuters/Ipsos poll released on Tuesday. Harris has led Trump in every Reuters/Ipsos poll since she entered the race in July, but her advantage has steadily shrunk since late September.
Throughout her campaign, Harris has tried to paint Trump as a threat to democracy who will infringe on the rights of Americans, including on the reproductive rights of women.
For his part, Trump has sought to tie Harris to Biden's handling of the economy - a top issue for voters, and one which voters have consistently given him an edge.
Former US President Barack Obama spoke for Kamala Harris in the key state of Pennsylvania. (EPA PHOTO)
High prices of food, utilities and housing have roiled voters, though the US economy has outperformed the rest of the developed world since the COVID-19 crisis.
Trump has also made his opposition to illegal immigration a central theme of his bid. At his event on Tuesday, which started over an hour late, he played a video highlighting a 12-year-old girl who was killed in Houston, allegedly by two Venezuelan men in the US illegally.
"In less than four years Kamala Harris has obliterated our borders," he said.
Trump is due later in the day to visit a heavily Hispanic city in Pennsylvania, two days after his rally in New York drew outrage because of an ally's vulgar and racist remarks.
Comedian Tony Hinchcliffe, who opened for Trump, called the Caribbean US territory of Puerto Rico a "floating island of garbage" and disparagedblack Americans, Jewish people and Latinos.
