WASHINGTON (BLOOMBERG) - Donald Trump said he's a "very stable genius," a day after a new book about the president's first year in the White House claimed that many of his top aides and confidants consider him unfit to hold office.

"Throughout my life, my two greatest assets have been mental stability and being, like, really smart. Crooked Hillary Clinton also played these cards very hard and, as everyone knows, went down in flames," Trump said on Twitter early on Saturday (Jan 6).

Trump is spending the weekend at the Camp David presidential retreat in rural Maryland, where he's in meetings with top Republican lawmakers, White House aides and several Cabinet members, including Secretary of State Rex Tillerson and Defence Secretary Jim Mattis, on legislative goals for the year and other pressing matters.

In the tweets, Trump added that he'd gone from being a very successful businessman to top TV 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!"

The comments follow the release of Michael Wolff's book, Fire And Fury, which details dysfunction, chaos and incompetence in the Trump White House - claims the administration has denied. Wolff said in an interview with NBC Friday that "100 per cent of the people around" the president question his intelligence and fitness for office.

PHYSICAL SCHEDULED

Trump, 71, is scheduled for a physical on Jan 12 at the Walter Reed National Military Medical Centre in Bethesda, Maryland, consistent with the practice of previous presidents.

In his Twitter messages, Trump said his detractors, including Democrats and the media, were shifting from stories about Russian collusion with members of his campaign team, which he's repeatedly denied, to focusing on his fitness for office.

Those critics "are taking out the old Ronald Reagan playbook and screaming mental stability and intelligence," Trump told his 46 million Twitter followers.

Reagan, the 40th US president, disclosed in November 1994, almost six years after leaving office, that he'd been diagnosed with Alzheimer's disease earlier that year. He died in 2004.

The publication of Wolff's book, initially via explosive excerpts focused on Trump's former chief strategist Steve Bannon and in its entirety on Friday, sapped the momentum with which the president entered 2018 after the passage of a Republican tax overhaul Bill in late December.

DEFENSIVE MODE

Instead of focusing on legislative priorities ahead of mid-term elections in November, Trump and the White House spent the week in defensive mode. That included a public falling out with Bannon, since nicknamed "Sloppy Steve" by the president, and the dispatch of a cease-and-desist letter demanding that Wolff's publisher, Henry Holt and Co. stop distribution of the book. The company instead moved up the publication date by several days.

This weekend's retreat will be a chance to refocus. The White House late on Friday distributed photographs of Trump flanked by Senate Majority leader Mitch McConnell and House Speaker Paul Ryan at a working dinner, and holding court at a meeting with lawmakers, aides and Vice-President Mike Pence.

On Thursday, Trump on Twitter called the book "phony" and "full of lies, misrepresentations and sources that don't exist."

Wolff has been making the rounds to discuss his work. Wolff, whose previous books include a biography of media mogul Rupert Murdoch, said he stands by everything in Fire And Fury and has notes and recordings to back it up.

"They all say he is like a child," Wolff told NBC. "What they mean by that is he has a need for immediate gratification. It's all about him."

"This man does not read, does not listen. He's like a pinball, just shooting off the sides," Wolff added. "They say he's a moron, an idiot."

"It's absolutely outrageous to make these types of allegations" about Trump's mental fitness, White House press secretary Sarah Huckabee Sanders said on Fox News on Friday.

They're "desperate attempts" to attack Trump, she said.

"What's I think is really mentally unstable" is people not seeing the progress the president is making.

While the president said he never spoke to Wolff for the book, the author said he spoke to Trump during the campaign and after the inauguration. "Whether he realised it was an interview or not," the conversations weren't off the record, Wolff said.

Wolff said that his sources told him Trump repeats stories over an increasingly short time period, and that he sometimes doesn't recognise longtime friends.

Trump's campaign released a letter from his doctor in September 2016, several weeks before the election, saying he was in "excellent physical health."

The upcoming physical will be his first known exam by a government doctor since taking office. Past presidents have also undergone periodic exams by government physicians who released statements afterwards.