Biographer says he sees 'clear deterioration' in Donald Trump's 'cognitive functioning'

David Cay Johnston -- author of The Making of Donald Trump -- said that President Trump is showing a "clear deterioration" in his mental faculties and that he is seeing this borne out in revelations from Michael Wolff's bombshell bookFire and Fury: Inside the Trump White House.

Johnston told SiriusXM Radio's Dean Obeidallah on Saturday that there are obvious signs that Trump is losing his grip on reality.

"I think I see a clear deterioration in Donald," the author said. "Back in 1990, for example, one of the very long conversations I had with him, he couldn't follow some basic tax advice I tried to give him to see how much he understood about taxes, which is nothing."

Today, Johnston said, "you look at the narrow vocabulary he's using" and his difficulty remembering people's names, along with the fact that he never exercises and eats "a godawful diet," it's clear that Trump is in decline.

"He does not have the cognitive function that he had in his 40s and 50s," he said. "That's very clear."

In the past, Johnston has called Trump a con-man and a racist and said that the president has no moral principles outside of winning and self-promotion.

Listen to the interview, embedded below: