WASHINGTON — The talk of a 2020 challenge from Oprah Winfrey comes as President Trump is defending his mental fitness for office. Author Michael Wolff, whose bestselling book portrays Mr. Trump as lacking the mental capacity for the presidency, maintains that even among those in the White House, there are questions.

“The people in the White House are just like everybody else in the country,” Wolff said in an appearance Monday on “CBS This Morning.” “What’s going to happen here? We don’t know from day to day.”

Wolff said there are times when members of the president’s inner circle are so alarmed by his unpredictability and temper that they wonder if they should take extraordinary measures.

“They don’t say, ‘The Cabinet is going to remove the president,'” Wolff said. “But they do say things like, ‘Well, this is a little 25th Amendment-y here.'”

Wolff was clear that he is not qualified to offer his personal view on Mr. Trump’s mental health. “I have merely described — and mostly not my impressions, the impressions of other people, of the people he deals with on a daily basis,” he said Monday.

Under the 25th Amendment, if a majority of a president’s Cabinet and the vice president agree that the president is mentally unfit, a president can be removed from office. However, the president objects, two-thirds of the House and Senate must support removal.

White House insiders flatly deny Wolff’s report that they talk about Mr. Trump’s mental fitness. Over the weekend, Mr. Trump described himself as a “very stable genius” and trumpeted his own mental acuity.

“I was a very excellent student, came out, made billions and billions of dollars, became one of the top business people, went to television and for 10 years was a tremendous success, which you’ve probably heard,” he said at a press conference with Republican leaders at Camp David Saturday. “Ran for president one time and won.”

Wolff’s book “Fire and Fury” was an immediate bestseller, and after explosive excerpts were released last week, it was released days early — on Friday. Wolff is not the first to raise questions about Mr. Trump’s mental health. In the 2016 book, “The Dangerous Case of Donald Trump,” numerous psychologists and therapists argue they have an ethical obligation to describe Mr. Trump as unfit for office.

“Delusional levels of grandiosity, impulsivity, and the compulsions of mental impairment, when combined with an authoritarian cult of personality and contempt for the rule of law, are a toxic mix,” the authors wrote.

Those psychologists and therapists admit they diagnosed from a distance, doing so, they said, out of an ethical obligation to warn the country.

Mr. Trump will receive his first physical as commander-in-chief Friday.  A psychological evaluation will not be part of the process.