Donald Trump attacked his rivals for the 2024 Republican presidential nomination and Democrat Joe Biden in an interview aired on Wednesday as counter-programming to his party's first primary debate.

Trump skipped the debate in Milwaukee, Wisconsin, and opted instead to do an interview with Tucker Carlson, a conservative former Fox News talk show host.

During the rambling pre-recorded interview, which aired on X at the same time as the debate, Trump said it did not make sense for him to take part while he was leading his rivals by "50 to 60 points" in the polls.

"Do I sit there for an hour or two hours, whatever it's going to be, and get harassed by people that shouldn't even be running for president?" he said.

Trump dismissed Florida Governor Ron DeSantis as a "lost cause," said he was "very disappointed" by his former vice president Mike Pence and called former New Jersey governor Chris Christie a "lunatic".

Trump touched on a wide range of subjects during the 45-minute interview which took place in a wood-paneled room at his Bedminster golf club in New Jersey, including being asked whether he thought Jeffrey Epstein, the US financier arrested for sex crimes, had killed himself.

"I think he probably committed suicide," Trump said.

Trump called Biden the "worst president in the history of our country" and suggested that the 80-year-old may not be the Democratic candidate on election day in November 2024.

"I don't think he's going to make it to the gate but you never know," Trump said. "I think he's worse mentally than he is physically and physically he's not exactly a triathlete.

"In many ways I'd love to run against him because his record's so bad."

Trump also dismissed his four criminal indictments. Trump has been indicted in New York on charges of paying hush money to a porn star, in Florida for mishandling top secret government documents and in Washington on charges of conspiring to upend his 2020 election loss.

Asked by Carlson what his priority would be if he wins re-election, Trump said "the first thing I would do is I would seal up the border good and tight except for people that want to come in legally." (AFP)