Washington DC - Kamala Harris will make her "closing argument" on Tuesday against rival Donald Trump, a campaign official said, a key speech to be delivered at the Washington site where the ex-president rallied supporters before the January 6 riot.

The vice president, a former prosecutor, will aim to draw a sharp contrast between her vision and that of Trump, who she says is sowing chaos and division, the Harris campaign official said Wednesday on condition of anonymity.

Trump supporters then marched on the Capitol, an assault on the seat of American democracy that left at least five people dead and 140 police officers injured.

Trump faces federal felony charges in Washington related to efforts to overturn the results of the 2020 election.

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The Harris campaign official said the symbolic location aims to spotlight how a second Trump term could go.

Trump was quizzed last week about the insurrection at the US Capitol, but he denied any responsibility, calling January 6, 2021, "a day of love."

Harris on Wednesday said Trump was "increasingly unhinged," and called her election rival's reported praise for Adolf Hitler "incredibly dangerous" as campaigning intensified ahead of the November 5 vote.

Harris said, "All this is further evidence for the American people of who Donald Trump really is."

Trump's camp hit back, saying Harris is "desperate because she is flailing, and her campaign is in shambles."

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With the tight election going down to the wire, both candidates are on a mission to persuade the sliver of American voters who remain undecided in the home stretch.

Since President Joe Biden's shock withdrawal, the Trump-Harris race has been one of the tightest in American history.

Past opinion polls have underestimated backing for Trump but also failed to predict the level of support for Democrats.