US President Donald Trump has rejected questions raised over his mental health in a bombshell book, describing himself as a "very stable genius".
Michael Wolff's book Fire and Fury: Inside the Trump White House suggests even those close to Mr Trump have questioned his capability.
The US president has dismissed the book, calling Wolff a "loser".
The row has overshadowed a key meeting of Republicans as they try to thrash out their priorities for 2018.
A retreat at Camp David will focus heavily on a strategy before crucial congressional elections in November.
Mr Trump is among those attending, but on Saturday the fallout from the book was again his focus, with the president accusing Democrats and the "fake news" media of "screaming mental stability and intelligence".
Skip Twitter post by @realDonaldTrump
….Actually, 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. I went from VERY successful businessman, to top T.V. Star…..
— Donald J. Trump (@realDonaldTrump) January 6, 2018
End of Twitter post by @realDonaldTrump
Skip Twitter post 2 by @realDonaldTrump
….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!
— Donald J. Trump (@realDonaldTrump) January 6, 2018
End of Twitter post 2 by @realDonaldTrump
Fire and Fury went on sale early on Friday, days ahead of its scheduled release, amid the president lawyers' attempts to block its publication. It has become an instant bestseller.
The book describes a Trump team shocked by their own win on election night, a chaotic White House, and senior administration officials calling Mr Trump an "idiot".
It has also sparked a public rift between Mr Trump and his former aide Steve Bannon, who is quoted as accusing Mr Trump's eldest son Donald Jr of "treasonous" behaviour in meeting a group of Russians.
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Mr Bannon and the author have both been the target of the president's ire over the past few days – the former cried when he lost his job last year, Mr Trump said; the latter had written a book "full of lies", he added.
On Friday, US Secretary of State Rex Tillerson told CNN that he had "no reason to question" Mr Trump's mental fitness.
Trump is not letting this one go
Analysis by Anthony Zurcher, BBC News Washington
Perceived slights, insults and questions about his intelligence. If Donald Trump's recent Twitter feed is any indication, these are the topics on the president's mind as he settles in for the night and when he rises in the morning.
Given the daunting tasks facing the administration and Congress in the coming weeks, some of his allies and aides at Camp David may view the president's concerns as misdirected.
That Mr Trump feels compelled to respond to criticism, however, should come as little surprise. This is particularly true when the topic is his intellect, the strength of which he frequently boasts.
In Fire and Fury, Michael Wolff recounts tales by White House aides of a president with a short attention span, bouncing from issue to issue like a pinball. In recent interviews, the author has said the president's aggressive reaction to his book proves this point.
It seems, however, that Mr Trump's counterattack is just getting started.
What is the Republican meeting about?
The Camp David summit begins two weeks before the end of Mr Trump's first year in office, and seeks to tie up unfinished business by Republicans.
"We have a lot of things to work on, a lot of things to accomplish," Mr Trump said as he left for the Maryland retreat.
The agenda has not been made public, but the key issues are likely to be:
Money: Or more specifically, how legislators can agree on funding the federal government for the current fiscal year. If they don't do so before 19 January, there is a risk of a government shutdown.
How to win in 2018: Congressional elections are 10 months away, with all 435 seats in the House of Representatives up for grabs, and another 33 in the Senate.
It will be difficult to avoid discussing the victory of Democrat Doug Jones in last month's Senate race in deeply conservative Alabama. Wider Democrat wins later in 2018 would make it much more difficult for Mr Trump and Republicans to push through their policies.
Immigration: Namely, what protection will be given to hundreds of thousands of undocumented immigrants who were brought to the US as children
The opioid crisis: These drugs killed more than 33,000 people in 2015, according to the Centers for Disease Control and Prevention. Mr Trump has promised to address the situation, but his "opioid czar" Kellyanne Conway does not appear to be at Camp David.
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