Donald Trump has questioned whether President Joe Biden will be physically capable of running for re-election in 2024 – as he skipped the first Republican debate of the campaign.

The former president made the comments as he took part in a 46-minute interview uploaded to X, formerly known as Twitter, on the same night as eight of his party colleagues clashed on TV in Milwaukee, Wisconsin.

But Trump said it would not have made sense for him to be involved in the debate, hosted by Fox News, because he was so far ahead in opinion polls in the race be the Republican Party’s candidate.

Instead, he spent much of the interview lashing out at Biden, who he described as “the worst president in the history of our country”.

Trump, 77, also claimed that Biden, 80, “can’t put two sentences together, can’t speak, can’t walk, can’t talk”.

He added: “I don’t think he gets to the starting gate, but these people do miracles.”

The former president, who was defeated by Biden in the 2020 presidential election, said his rival was “worse mentally than he is physically and physically he’s not exactly a triathlete… You’re waiting for him to collapse and he almost always does”.

But Trump said he would still “love” to run against the current occupant of the White House, who he dubbed “crooked Joe Biden” – similar to the nickname he used for Hillary Clinton during the 2016 election campaign.

However the Republican hopeful is facing four separate sets of criminals charges and it came as his former lawyer Rudy Giuliani surrendered to authorities in Georgia on Wednesday on charges of trying to overturn the results of the 2020 election.

But Trump dismissed the various allegations as politically motivated – and said he was confident that “the people see it’s a fraud”.

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The interview was conducted online by controversial former Fox News host Tucker Carlson, who left the US network earlier this year.

Trump declined to be drawn on Carlson’s suggestion that he could be assassinated by political opponents, and was also reluctant to agree with the host’s suggestion that financier Jeffrey Epstein, who died in prison in August 2019, was murdered.

When asked if he thought Epstein had been killed, Trump replied: “It’s possible, [but] I don’t really believe – I think he probably committed suicide…

“But a lot of people think he was killed. He knew a lot on a lot of people.”

But Trump was enthusiastic in lashing out at other rivals, including Republican presidential hopefuls Asa Hutchinson – who he described as “weak and pathetic” – and former ally Chris Christie – who he said was “like a lunatic”.

Explaining his decision not to take part in the debate, he said: “I’m leading by 50, 60 points, and some of them [his opponents] are at one and zero…

“Do I sit there for an hour, two hours, whatever it’s going to be, and get harassed by people that shouldn’t even be running for president?”

He added: “I’m going to have all these people screaming at me, shouting questions at me, all of which I love answering, I love doing, but it doesn’t make sense to do them, so I’ve taken a pass.”

Meanwhile Trump also called for the war in Ukraine to be ended “immediately”.

He claimed: “if I were president, it would never [have] started.”