Ms Trump – the president’s daughter-in-law, married to Eric Trump – said the former vice president and Democratic challenger’s behaviour is “very concerning to a lot of people" but she claimed she had “no idea” he has a stutter.
During an Iowa campaign event, Ms Trump said: "I feel kind of sad for Biden ... I'm supposed to want him to fail at every turn, but every time they turn to him I'm like, 'Joe, can you get it out? Let's get the words out, Joe.’”
The president’s re-election campaign has spent months doubting Mr Biden’s mental acuity, while the president has also attacked his opponent directly, suggesting he “can’t put two sentences together” and has dementia.
In June, a Fox News reporter asked Mr Biden if he had “been tested for some degree of cognitive decline.”
“I can hardly wait to compare my cognitive capability to the cognitive capability of the man I’m running against," he replied.
The Trump campaign also recently held a call with “former White House physician” Dr Ronny Jackson, who spoke at length about Mr Biden’s cognitive abilities. But when pressed whether he was making a medical assessment, he said: “I'm not making a medical assessment. I actually don't even practice medicine at this point.”
On CNN, Ms Trump said she had “no idea that Joe Biden ever suffered from a stutter."
“I think what we see on stage with Joe Biden, Jake, is very clearly a cognitive decline," she said.
He replied: "I think you have absolutely no standing to diagnose someone’s cognitive decline … I would think that somebody in the Trump family would be more sensitive to people who do not have medical licenses diagnosing politicians from afar. Plenty people have diagnosed your [the president] from afar. And I’m sure it offends you.”
“I’m not diagnosing him,” she said.
“You just talked about cognitive decline,” he responded.
“It’s very concerning to a lot of people,” she said.
"This could be the leader of the free world. That is all I’m saying. I genuinely feel sorry for Joe Biden.”