US President Donald Trump on Thursday called Kamala Harris, who is making US history as the first Black woman vice presidential candidate, a "monster."

Trump, far behind in the contest for the White House on November 3 and doing especially badly in polling of women voters, used the word twice, calling the Democrat "this monster."

Speaking to Fox Business News for almost an hour in his first interview since falling sick with the coronavirus, Trump said Vice President Mike Pence "destroyed" Harris in their debate Wednesday in Utah.

"Everything she says is a lie," Trump said.

Harris, who is also the first person of Indian descent to be on a White House ticket, spent most of her debate attacking Trump for presiding over what she said was "the greatest failure of any presidential administration in the history of our country" over Covid-19.

In the Fox interview, Trump once more branded Biden, who is leading handily in almost every significant opinion poll around the country, "not mentally capable."

"Everybody knows that," Trump claimed, predicting "Biden won't be president for two months."

Trump, who for months has slurred Biden as senile and previously called Harris "nasty," also revived one of his darkest claims as a politician -- that illegal Latin American immigrants equate to serious criminals.

Democrats "want to have thousands of people that are murderers, rapists, just pour into our country. They could be very sick people," he said.

Similar descriptions of undocumented migrants have been a staple of Trump's brand ever since he announced his presidential run in 2015 with the statement that Mexicans coming to the United States are criminals, "rapists and some, I assume, are good people."