Trump claims 'something is wrong' with Joe Biden and compares himself to a chess grandmaster

President claims it would be 'impossible to have a fair election' if 51m mail-in ballots are sent out

Donald Trump, counter-programming the Democratic National Convention, told Fox News "something is wrong with Joe Biden" that leaves the party's soon-to-be presidential nominee unable to "fight back" against leftist forces.

"They've forced him to support things that no one ever would have thought," the president told Sean Hannity of the progressive wing of the Democratic Party. "And he's not in a position to fight back. ... Something's going on. I assume we'll be able to find out ... sooner rather than later.

A few minutes later, the 74-year-old Mr Trump compared himself to a world-class chess player, contending Mr Biden lacks the mental capacity at 77 to stand up to the world's harshest leaders, like Russian President Vladimir Putin and North Korea's Kim Jong Un.

"If you are not at the top of your game, it's not going to be a pretty picture," he said. "It's like: chess masters. And if you are not a great chess master, you're gonna have a problem."

Mr Hannity, without providing any medical evidence or an expert to speak to his observation, made this claim about Mr Biden: "He seems frail to me."

The president, also without providing evidence, replied: "It's pretty obvious what's going on."

He did not offer up the diagnosis he obtained from a medical professional, however.

The president used much of the first half of of the 30-minute phone interview to try painting Mr Biden, set to accept the Democratic nomination about an hour later, as too old to replace him.

He also contended that if states send out 51m mail-in ballots, it would be "impossible to have a fair election".