Trump challenges Biden to a cognitive test as the US President calls his predecessor a felon

A halting Joe Biden struggled Thursday to allay concerns he is too old for a second term in the White House in a fiery debate with Donald Trump marked by personal insults.

A bombastic Trump lashed out at his successor, calling him a failure on the economy and the world stage.

Biden looked to hit back, but his delivery was faltering as he spoke rapidly in a raspy, trailing-off voice, stumbled on his words and stared open-mouthed.

His performance, after he spent the week secluded in preparation, sparked new concern within his Democratic Party as polls show Trump is tied or ahead for the November election.

It was the first debate ever between a president and former president -- and each accused each other of being history's worst.

Trump and Biden, who were each the oldest president when first elected, even accused each other of being childlike as they argued over their golf swings.

Biden, 81, and Trump, 78, did not shake hands as they walked to their podiums at the CNN headquarters in Atlanta. There was no live audience and their microphones were muted as the other spoke.

Biden hit Trump with clearly rehearsed lines as he sought to remind millions of television viewers that Trump would be the first convicted felon in the White House.

"You have the morals of an alley cat," Biden said.

Trump, a veteran of rallies and reality television, spoke loudly as he ran through a long list of complaints about Biden's record.

"I'm friends with a lot of people. They cannot believe what happened to the United States of America. We're no longer respected," Trump said.

Trump sought to seize on Biden's delivery, saying at one point, "I really don't know what he said at the end of that sentence. I don't think he knows what he said either."

Trump further challenged Biden to take a cognitive test, which he claimed the President would fail.

Also watch: Biden, Trump begin high-stakes US presidential debate