Vice President Kamala Harris has labeled Donald Trump a "fascist" after his longest-serving chief of staff said the former president praised Adolf Hitler while in office.

In a public forum hosted by CNN's Anderson Cooper, he asked Harris if she thinks Trump is a fascist. "Yes, I do," she said Wednesday.

The question follows remarks from Trump's former chief of staff, John Kelly, who told the New York Times that in his view Trump "certainly falls into the general definition of fascist, for sure."

He referred to the description of fascism as an extreme right-wing, authoritarian and ultra-nationalistic ideology, characterized by a dictatorial leader and the suppression of opposition.

Harris, 60, said that Trump is "increasingly unstable" and "unfit to serve." She stated that former employees of the ex-president and close associates have explicitly pointed out that Trump despises the United States Constitution and should never again hold the office of president of the United States.

Kelly, she said, had issued an emergency "call to the American people" with his statements about Trump, which highlighted what could happen if he returns to the White House.

The people who could "hold Trump back" are now no longer there. "I believe Donald Trump is a danger to the well-being and security of America," said Harris.

The election campaign in the United States has reached its final stage, with the final vote to take place on Nov. 5. Millions of US citizens have already cast their ballots in early voting. Harris and Trump are in a dead heat, according to polls.