Former President Donald Trump is "unstable", "obsessed with revenge" and is "out for unchecked power", Vice President and Democratic candidate Kamala Harris said on Friday in an unprecedented attack on her Republican rival ahead of next week's US election.

In a rally at Las Vegas, 60-year-old Harris, who is of Indian and African heritage, said Trump, 78, is all about "hate" and more division and that he would bring an "enemies list" to the White House, while she would carry a "to-do list" of priorities.

Singer Jennifer Lopez joined the Democratic leader at the rally and called on voters, especially Latino Americans, to support Harris as cares about "not just some Americans, but all Americans".

"We all know who Donald Trump is. He is not someone who is thinking about how to make your life better. He is someone who is increasingly unstable, obsessed with revenge, consumed with grievance and he is out for unchecked power," Harris said at the rally.

With just four days left for the November 5 US presidential election, the race between Trump and Harris remained very tight in the key battleground states.

"If he (Trump) is re-elected, Donald Trump will walk into the office with an enemies list. If I am elected, I will walk in with a 'to-do list' on behalf of you," Harris said.

"And at the top of my list is bringing down the cost of your living. That will be my focus every single day as the President," she added.

Harris said she will put the country ahead of her party and will be a president for all Americans.

"Unlike Donald Trump, I do not believe that people who disagree with me are the enemy. He wants to put them in jail. I want to give them a seat at the table. That's what real leadership looks like," she said.

"I pledge to always put country above party, and self and to be a president for all Americans," she added.

Harris said, if elected to the White House, she will make housing more affordable by building more houses. "We will cut taxes for small businesses, we will lower health care costs." "I see the promise of America in everyone here," she said.

The Democratic leader attacked Trump for his comment that he would protect women whether they "like it or not".

The comments showed that Trump "does not understand women's rights to make decisions about their own lives, including their own bodies".

The presidential election race remained razor-tight. The latest CNN polls showed that Harris has a narrow advantage in Michigan and Wisconsin, while she and Trump remain tied in Pennsylvania.

In Michigan, Harris holds a lead over Trump by 48 per cent to 43 per cent among likely voters, according to the findings Harris also leads Trump in Wisconsin, where 51 per cent support her to 45 per cent for Trump, but the race is tied in Pennsylvania with both having 48 per cent support.

More than 62 million voters have cast early ballots, according to the University of Florida’s Election Lab.

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