Portraits of Theodore and Franklin Roosevelt loomed watchfully. Five US senators with decades of collective experience stood in deference at their chairs. Donald Trump was about to enter the room, a prospect that assures a frisson of unpredictability.

The president strode in, shook hands with Chuck Grassley and patted Lindsey Graham on the back. Graham playfully punched the air.

“Lindsey used to be a great enemy of mine,” Trump told the gathering when all were seated a few minutes later, “and now he’s a great friend of mine.”

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The senator for South Carolina shifted awkwardly in his chair and grinned: “I like me too, so we have something in common.” There was a ripple of laughter.

Later, when 71-year-old Trump turned to Senator James Lankford and called him “Tom”, everyone pretended to ignore it.

Here in the elegantly appointed Roosevelt Room, cocooned in the West Wing as if in a wartime bunker, Trump and the loyalists who lavished praise on his leadership could maintain the pretence of business as usual. But outside, political fires were burning. The White House had been hit by a bombshell book that portrayed it as a hive of discord, dysfunction and farce, and the president himself as ignorant, capricious and clinically unfit for the office once occupied by the Roosevelts.

Fire and Fury: Inside the Trump White House, by Michael Wolff, a devastating fly-on-the-wall account of life at 1600 Pennsylvania Avenue, has stirred speculation over the mental health of the man who began the week boasting to the world of the size of his nuclear button. “His stability,” Carl Bernstein, a veteran Washington Post reporter best known for his work on the Watergate scandal, told CNN. “That’s really what this book is about.”

Play Video 1:18 Fire and Fury: Key explosive quotes from the new Trump book - video

Wolff, who parked himself for long hours on a West Wing sofa, says he interviewed more than 200 people in Trump’s inner and outer orbit and they reached a joint conclusion.

“They all say, ‘He is like a child,’ and what they mean by that is he has a need for immediate gratification,” the author told NBC on Friday. “It’s all about him.”

On Saturday, he spoke to the BBC. “I think one of the interesting effects of the book so far is a very clear ‘emperor has no clothes’ effect,” Wolff said.

“The story that I have told seems to present this presidency in such a way that it says he can’t do his job.”

‘Treasonous’ and ‘unpatriotic’

The new year had barely rubbed the sleep from its eyes when, on Wednesday morning, the Guardian published excerpts of Wolff’s book a week ahead of its scheduled publication. It vividly depicted chaos and conflict as a default setting in the administration, even worse than many already suspected. At the centre of the palace intrigue was Steve Bannon, the hardline nationalist who helped put Trump in the White House.

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Bannon, former chief strategist, had spilled the beans to Wolff with stunning candour. He described the decision by Trump’s son, Donald Trump Jr, to meet a group of Russians at Trump Tower during the 2016 election campaign as “treasonous” and “unpatriotic”. He predicted that the continuing investigation into alleged collusion with Moscow would run and run: “They’re going to crack Don Junior like an egg on national TV.” And he called Trump’s daughter Ivanka “as dumb as brick”.

Bannon was not alone in his unflattering verdict. The book also reported that Rupert Murdoch once mocked Trump as “a fucking idiot” over his incoherent views on immigration policy, while Thomas Barrack Jr, a billionaire who is one of the president’s oldest associates, allegedly told a friend: “He’s not only crazy, he’s stupid.” (Barrack has since denied this.)

Other claims by Wolff include Melania Trump’s reaction to an election win that her husband and his team thought impossible – she was “in tears – and not of joy”; the couple’s preference for separate bedrooms; and Trump’s habit of going to bed with a cheeseburger at 6.30pm, watching three TV screens and making phone calls. The president reprimands housekeeping staff “for picking up his shirt from the floor” and “imposed a set of new rules: Nobody touch anything, especially not his toothbrush”. His paranoia about being poisoned leads him to turn up without warning at McDonald’s, according to the book, which Trump has branded “boring and untruthful”.

The barrage of damning revelations caught the White House off guard. The Washington Post reported: “Trump spent much of the day raging about the book to top aides, officials and advisers said … As he fumed, some aides were still frantically searching for a copy of the book, and even senior aides like [Hope] Hicks had not seen it by the afternoon, officials said.”

Play Video 0:39 Donald Trump has 'lost it', says Michael Wolff – video

Said by officials to be “disgusted” and “furious”, Trump launched an abortive legal attempt to block the book, which doubtless boosted sales, and wasted little time in brutally disavowing Bannon. Usually he is content to hammer foes with a vituperative tweet – not this time. The White House released a bilious 266-word statement that played down Bannon’s role in Trump’s electoral success and declared: “When he was fired, he not only lost his job, he lost his mind.”

It was an ugly, very public political divorce, the end of a relationship that had transfixed the political world. In August 2015, Bannon, a former naval officer, investment banker and film-maker, boasted that he had turned the rightwing website Breitbart News into “Trump Central” and joked that he was the candidate’s hidden “campaign manager”, the New York Times reported. He hosted Trump for friendly radio interviews and, in August 2016, took over as chief executive of his long-shot presidential campaign.

The scruffy, unshaven Bannon came to personify Trump’s darkest, nativist impulses on immigration, building a border wall and threatening a trade war; he fanned the flames of “America first”, white nationalism and a blow-everything-up philosophy. In a November 2016 interview with Wolff for the Hollywood Reporter, the liberal bete noire boasted of his influence, declaring with relish: “I am Thomas Cromwell in the court of the Tudors.” It was grimly prophetic – Cromwell was eventually executed on the king’s orders.

‘Sloppy Steve’

Facebook Twitter Pinterest The book chronicles feuding between Steve Bannon, second from left, and Jared Kushner, second from right. Photograph: Mandel Ngan/AFP/Getty Images

Bannon made two fatal mistakes. One was overreach: he featured on the cover of Time magazine, was portrayed as more powerful than Trump on Saturday Night Live and was dubbed “President Bannon”, all bound to infuriate the Trump ego. He also displayed visceral hatred for Ivanka and her husband, Jared Kushner, chronicled by Wolff in lurid detail. After seven months, he was ousted as chief strategist and returned to his perch as executive chairman of Breitbart.

He reportedly continued to speak to Trump by phone; officials say the last such conversation happened in early December. Around the same time, the Bannon-endorsed Roy Moore – accused of sexual misconduct involving teenage girls – lost a US Senate special election in the Republican stronghold of Alabama, casting doubt on Bannon’s so-called strategic genius.

Now, Bannon has not only lost Trump. Soon after, he lost his patron Rebekah Mercer, the billionaire Republican donor, who turned off the financial tap. If, as is speculated, he loses Breitbart too, he could be banished to the political wilderness. Reader comments on Breitbart’s site seemed overwhelmingly supportive of the president. The alt-right, of which Bannon was once the rock star, appeared to be uniting around Trump.

In a tweet on Friday night, Trump said Bannon “cried when he got fired and begged for his job. Now Sloppy Steve has been dumped like a dog by almost everyone. Too bad!”

But Kurt Bardella, a political commentator and former Breitbart spokesman, predicted that Bannon may prove difficult to oust from the organisation, and may yet use it as a platform to get back into the president’s good graces.

“If there’s one thing we know about Donald Trump it’s that he’s susceptible to being sucked up to,” Bardella said. “The ego of Trump would love nothing more than a humbled Steve Bannon coming grovelling for his forgiveness. He may have imagined just that scenario.”

On Thursday, Bannon called Trump a “great man”, but reconciliation still seems a long way off. His demise has been embraced by the Republican establishment, which had feared an insurgent movement by Bannon-backed candidates with an “America first” agenda ahead of November’s midterm elections. Recently, Bannon warned the Senate majority leader, Mitch McConnell: “You’re like a deer that’s been shot, you’re just going to bleed out, brother.” After the White House released Trump’s statement denouncing Bannon as a self-aggrandising hanger-on who had “lost his mind”, McConnell’s campaign account tweeted a gif of the senator with a victorious smirk forming on his lips.

Bardella said: “Steve Bannon’s crusade for 2018 is over. Instead of Republican versus Republican, the general election between Republicans and Democrats starts now.”

He warned McConnell of a mixed blessing, however: “You may have sidelined Steve Bannon but you still have to deal with Donald Trump. Republicans are going to be responsible for whatever’s going to happen going forward. They’re not going to be able to say that there were no signs of diminished capacity. They’re not going to be looked on kindly by history.”

‘The danger has become imminent’

Facebook Twitter Pinterest Michael Wolff on the set of NBC’s Today show. Photograph: Brendan Mcdermid/Reuters

Wolff’s book has revived swirling speculation about the state of Trump’s mental health and the 25th amendment, which allows a majority of the cabinet and vice-president to remove the president by deeming him “unable to discharge the powers and duties of his office”.

The Trump who emerges from Wolff’s account is chronically incurious, contemptuous of experts and beholden to his gut instincts. “Trump didn’t read,” he writes. “He didn’t really even skim … He could read headlines and articles about himself, or at least headlines on articles about himself, and the gossip squibs on the New York Post’s Page Six.” Some allies tried to explain this away as an attribute of his populism. “He was postliterate – total television.”

In another passage, Wolff recounts how Trump is repeating himself with greater frequency. Whereas he used to repeat, “word-for-word and expression-for-expression”, the same three stories every 30 minutes, now it is within 10 minutes. And in another anecdote, he wrote in a column for the Hollywood Reporter: “At Mar-a-Lago, just before the new year, a heavily made-up Trump failed to recognise a succession of old friends.”

Bandy Lee, an assistant clinical professor at the Yale School of Medicine, briefed a dozen members of Congress last month on the potential risks associated with the president’s behavior. Lee said this week she and other psychiatrists were speaking out because they feel “the danger has become imminent”.

Democratic congressman Jamie Raskin, who attended Lee’s presentation and has proposed legislation to create a commission that would determine whether the president is mentally fit for office, said Trump’s behaviour was “increasingly delusional”. He told CNN: “It’s a very dangerous and unstable situation as a number of Republican senators have themselves observed”. No Republican in Congress has yet called publicly for an evaluation of Trump’s mental state.

Chris Ruddy, a longtime friend of Trump, believes the president has attention deficit disorder. “He’s easily bored,” he told the Observer. “He likes moving around a lot of topics and asking a lot of questions. He’s so smart, he’s easily bored. It’s a big leap to say he’s psychologically unfit for office.”

Ruddy said he saw Trump several times over the Christmas holidays. “I think he’s mentally fit. I didn’t see anything untoward. His conversation was consistent with any time I’ve known him. He seemed to recognise everyone around us. I brought a New York Times journalist, Michael Schmidt, to meet him and at first he didn’t recognise him but, when he did double take, he realised who it was.”

He added: “I would say Donald Trump’s mental acuity in remembering faces and information is much higher than average.”

Ruddy, chief executive of the conservative Newsmax Media, accused liberals of trying to use the issue to overturn the 2016 presidential election result. “It’s a highly politicised time and they’re trying to weaponise psychology and psychiatry. It makes no sense.”

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Trump made a similar argument in extraordinary tweets from 7.19am on Saturday, accusing Democrats and the mainstream media of “taking out the old Ronald Reagan playbook”. Reagan, whose record Trump broke as the oldest person to be elected president, came under scrutiny from opponents for forgetfulness and contradicting himself. Five years after leaving office, he was diagnosed with Alzheimer’s disease.

In comments unlikely to have the desired effect of quelling speculation over his own health of mind, Trump continued: “Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart... I went from VERY successful businessman, to top T.V. Star.... to President of the United States (on my first try). I think that would qualify as not smart, but genius....and a very stable genius at that!”

Debate over a president’s erratic behaviour often invites comparisons with Britain’s King George III. But Wolff’s portrayal of a man prone to wild mood swings, liable to call someone a friend one moment, an enemy the next, is also redolent of actor Forest Whitaker’s portrayal of Ugandan dictator Idi Amin in the film The Last King of Scotland.

“You’re a child,” his despairing doctor tells him. “You have the mind and ego of an angry, spoiled, uneducated child. And that’s what makes you so fucking scary.”