Appearing on PBS Newshour Monday evening, former White House aide Omarosa Manigault-Newman explained why she claims in a new book that President Donald Trump has cognitively declined in the last decade and a half and that he now has a "mental impairment."

Manigault-Newman, who first worked with Trump on his show "The Apprentice" before joining his campaign and then the White House, discussed the differences between Trump as he was when she first worked with him compared to his performance in the Oval Office.

"Donald Trump was sharp, he was perceptive, he was engaging," she said, referring to his work on the reality TV show. "He had this expansive vocabulary, and he very seldomly took breaks. I mean, he could go four or five hours, without blinking. Fast-forward to 2017, and we're in the White House, and Donald Trump couldn't remember basic words or phrases. He couldn't read the legislation that was put in front of him."

Host Judy Woodruff asked the guest when exactly she saw evidence of this behavior.

"I was in the room, in the Oval Office, trying to brief him, for instance, when we were getting ready to pass the executive order for [historically black colleges and universities]" she said. "Or briefing him for that famous listening session where he thought Frederick Douglass was still alive."

She added: "I'm not a doctor. I can only assess the Donald Trump that I knew in 2003 and the Donald Trump that I knew in 2017. And he is not the same man. In the morning he would say one thing, by the afternoon, he was contradicting himself. And he wouldn't remember that he was contradicting himself.

She also pointed out that Trump recently encouraged Republicans to pass an immigration bill, and then shortly thereafter, he claimed he never wanted them to pass the legislation.

"I don't believe that that's just him lying," she said. "I really do believe that he has some sort of mental impairment and decline."

Since Manigault-Newman began promoting her book, the White House has lashed out and called her deceitful and attention-seeking — which, of course, raises questions about why she was hired in the first place. Some people mentioned in her book, including GOP pollster Frank Luntz and Trump's former lawyer Michael Cohen, have directly contradicted claims she made about them. While her credibility is open to serious doubts, some of the incidents she cites as evidence of cognitive decline — including the Frederick Douglass remark and the contradictory claims about an immigration bill — happened in public view.

Watch the clip below: