President Joe Biden called his Republican challenger Donald Trump the main threat to US democracy in an interview aired Tuesday on the main US Spanish-language TV network.

Asked by Univision in the Oval Office what he considered the "primary threat to freedom and democracy at home," Biden said: "Donald Trump. Seriously."

Biden referred to Trump's support for the mob that stormed the Capitol on January 6, 2021, inflicting "destruction and mayhem" in an attempt to overturn his election loss to Biden two months earlier.

Trump is making praise for what he calls the January 6 "patriots" central to his current election campaign, saying that if elected in November he will issue pardons to the hundreds sentenced to prison for crimes including attacking police and seditious conspiracy.

Biden referred to congressional testimony that Trump had watched the riot for several hours on live television from the White House without attempting to intervene.

"The idea that he would sit in the office… and watch for hours the attack on the Capitol," Biden said, adding that Trump "uses phrases like you're gonna... eviscerate the Constitution (and) he's going to be a dictator on day one."

"I can't think of any other time, in my lifetime, in history that's occurred, that you’ve had somebody who's had this kind of attitude," Biden said.

Trump describes the United States as "failing" and suffering an invasion of violent immigrants, saying that only his return to the White House can save the country.

Although at first almost universally castigated by Republicans for the attack on the Capitol and his refusal to accept the 2020 election result, Trump has since returned to near total dominance of his party and polls show him in a tight race with Biden.

The Univision interview was a rare one-to-one interview for Biden, who has taken far fewer questions from journalists in the format than other recent presidents.

Hispanics are a key electoral target for both Biden and Trump in the November vote, and Univision is the country's leading Hispanic media company.