“Trump's tweets in his first hundred days draw a pretty clear portrait: he's an almost textbook case of narcissistic personality disorder,” Mr King wrote on Twitter.
He added: “That this guy has his finger on the nuclear trigger is worse than any horror story I ever wrote.”
Mr King has been a staunch critic of Mr Trump, particularly regarding his lack of financial transparency.
In April, he wrote: “Come on, chickens***, if you're man enough to drop a mega-bomb on Afghanistan, you should be man enough to release your tax returns.”
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Mr King was one of hundreds of authors who signed an open letter opposing Mr Trump during his election campaign.
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