Author Michael Wolff rebutted criticism Friday of his bombshell book on President Donald Trump's White House, saying many of the president's close advisors question his fitness for office.

In an interview with NBC's "TODAY," Wolff contended that Trump has "less credibility than, perhaps, anyone who has ever walked on Earth." Wolff insisted that he talked to the president for the book "Fire and Fury: Inside the Trump White House," despite Trump's denial on Thursday night in which he called the book "phony."

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The book features explosive quotes from former White House chief strategist Steve Bannon deriding Trump's children, as well as numerous claims of top advisors and friends calling the president terms like "idiot" and "dumb as s--t." It got released on Friday, four days earlier than originally planned, after early excerpts and Trump's attempt to stop it from getting published sparked massive interest.

Wolff said Friday that Trump's advisors and even family members questioned the president's ability to do the job.

"I will tell you the one description that everyone gave — everyone has in common — they all say he is like a child," the writer said. "And what they mean by that is he has a need for immediate gratification. It is all about him."

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He added that "senior advisers, family members, every single one of them, questions his intelligence and fitness for office." Wolff says the book is based on more than 200 interviews with people around the president.

In a tweet Friday morning, Trump called the book a "phony" attempt to discredit him. On Thursday, White House press secretary Sarah Huckabee Sanders labeled it "trash," "fraudulent" and "tabloid gossip."

Read the full NBC report here.

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