Washington DC - Vice President Kamala Harris on Wednesday said Donald Trump is "increasingly unhinged" and called his praise for Adolf Hitler, reported by a former top aide, "incredibly dangerous."

The Democrat's ferocious criticism of the ex-president came with just 13 days left in their tense White House race.

She spoke outside her residence in Washington after Trump's former presidential chief of staff, retired Marine general John Kelly, went public with revelations about the Republican's admiration for the Nazi dictator and his military in World War II.

"It's deeply troubling and incredibly dangerous that Donald Trump would invoke Adolf Hitler, the man who is responsible for the deaths of six million Jews and hundreds of thousands of Americans," Harris said before leaving for a televised town hall in swing state Pennsylvania.

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

Kelly told The New York Times that Trump had remarked that "Hitler did some good things too" and that instead of the US military, he "wanted generals like Adolf Hitler had."

Kelly also repeated his warning that he believes his ex-boss "certainly falls into the general definition of fascist."

Harris said, "Donald Trump is increasingly unhinged and unstable, and in a second term, people like John Kelly would not be there to be the guardrails against his propensities and his actions."

"So the bottom line is this: We know what Donald Trump wants. He wants unchecked power," Harris added, warning that Trump wanted a military that "is loyal to him" and not to the US constitution.