Michael Wolff also stood by his account that many in the administration think Trump is incapable of meeting the demands of his role – claims which have prompted a furious backlash from the White House.
The author noted testimony that Trump keeps repeating himself: whereas once he would tell the same three stories in 25 or 30 minutes, he said, now he does so in 10.
“I will quote Steve Bannon,” Wolff said. “‘He’s lost it.’”
Discussing the book on NBC’s Today show on Friday, in his first interview since the publication of excerpts by the Guardian triggered a political firestorm, Wolff asked: “Where do I send the box of chocolates?
“Not only is he helping me sell books, but he’s helping me prove the point of the book.
On Thursday Trump’s lawyer demanded a halt to the publication of Fire and Fury: Inside the Trump White House or excerpts. Its publisher instead brought forward the release date, generating excitement for politics junkies that earned comparisons with the launch of a Harry Potter book.
Kramerbooks, a popular Washington store just a mile from the White House, began selling copies at midnight; it was reported that all 75 had been sold by 12.15am. The book also shot to No 1 on Amazon’s bestseller list.
Trump attacked Wolff on Thursday evening, tweeting: “I authorized Zero access to White House (actually turned him down many times) for the author of the phony book! I never spoke to him about the book. Full of lies, misrepresentations and sources that don’t exist.”
Wolff, a media critic and columnist, responded: “What was I doing there if he didn’t want me to be there? I absolutely spoke to the president. Whether he realised it was an interview or not, I don’t know, but it certainly was not off the record.”
The author spent about three hours with Trump during the election campaign and in the White House, he continued, “so my window into Donald Trump is pretty significant”.
He added: “I spoke to people who spoke to the president on a daily, sometimes minute-by-minute basis. In a sense, there was one question on my mind when I began this book: what is it like to work with Donald Trump, how can you work with Donald Trump, and how do you feel having worked with Donald Trump?”
Asked how he gained such extraordinary access, Wolff said: “I certainly said whatever was necessary to get the story.”
White House says Wolff never interviewed Trump for book – video
The White House has dismissed the book as “tabloid gossip” full of “lies” and some commentators have cast doubt on Wolff’s credibility. Wolff said like any journalist he has recordings and notes and is “in every way comfortable” with everything he reported.
“My credibility is being questioned by a man who has less credibility than perhaps anyone who has ever walked on earth at this point,” he said.
Asked if he stands by his work, Wolff replied: “Absolutely everything in the book.”
A recurring theme in Fire and Fury is the question of Trump’s fitness for office. Wolff suggested that even the president’s son-in-law, Jared Kushner, and daughter, Ivanka Trump, have lost faith.
“Certainly Jared and Ivanka, in their current situation, which is a deep legal quagmire, are putting everything on the president,” he said. “‘Not us, it’s him.’”
Wolff added: “I will tell you the one description that everyone gave, everyone has in common. They all say, ‘He is like a child,’ and what they mean by that is he has a need for immediate gratification. It’s all about him. He just has to be satisfied in the moment.
“They say he’s a moron, an idiot. Actually, there’s a competition to sort of getting to the bottom line here of who this man is. Let’s remember, this man does not read, does not listen, so he’s like a pinball just shooting off the sides.”
White House press secretary Sarah Huckabee Sanders listens to remarks by President Donald Trump during Thursday’s daily press briefing. Photograph: UPI / Barcroft Images
The White House has pushed back, the press secretary, Sarah Sanders, saying on Thursday such claims were “disgraceful and laughable”. On Friday, a longtime friend and confidant of Trump rejected Wolff’s suggestion.
Chris Ruddy, chief executive of Newsmax Media, told CNN: “He is not psychologically unfit, he has not lost it… I saw the president every other day over a 10-day period during the holiday. I conversed with him during this time. I saw him interact with people. He was remembering things, he was on point, he was following up on discussions.”
Ruddy recalled that he took Michael Schmidt, a New York Times journalist, to interview the president at his Mar-a-Lago estate in Florida over Christmas.
“I don’t believe Michael walked out and said, ‘This man is crazy, this man is unfit’. So this is just an absurd allegation by someone who’s talked to a lot of disgruntled people at the White House.”
Rex Tillerson, whose future as secretary as a state has been subject to intense speculation, denied that he has personal concerns over the president’s condition. “I’ve never questioned his mental fitness,” he told CNN. “I have no reason to question his mental fitness.”
Amid such backlash, Steve Bannon, Trump’s campaign chief executive and former White House strategist who is a major source for Wolff’s book, appears increasingly isolated.
On Thursday night his principal financial backer, billionaire Rebekah Mercer, publicly withdrew her patronage – a severe blow to Bannon’s plans to back a slate of candidates in the midterm elections. The White House suggested Breitbart News should “look at and consider” ousting Bannon as executive chairman.
On Friday, before departing for a retreat with Republican leaders at Camp David, Trump tweeted: “The Mercer Family recently dumped the leaker known as Sloppy Steve Bannon. Smart!”