President Donald Trump lashed out at critics Saturday in defense of his mental fitness for office, calling himself a "very stable genius" in a tweetstorm of boasts.

First on Twitter, then at a news conference with Republican leaders at Camp David, Trump defended himself against a new book that cites purported fears from former and current aides that he was unprepared for the presidency, incapable of processing information and uninterested in making difficult decisions.

Citing his success in business and on television, as well as his victory in presidential politics on "my first try," Trump tweeted that his record "would qualify as not smart, but genius....and a very stable genius at that!" He suggested that the "Fake News Mainstream Media" are trying to smear him by using the "playbook" on President Ronald Reagan, who some believed suffered from mental deterioration due to age in the latter years of his two terms. Reagan was diagnosed with Alzheimer's disease after leaving office.

During a news conference at the presidential retreat in Maryland, where Trump and GOP leaders were formulating their 2018 agenda, the president denounced the book's author, New York media writer Michael Wolff, and a high-profile source, former White House chief strategist Stephen Bannon. Trump, whose personal lawyer sent a cease-and-desist letter in an effort to stop publication of "Fire and Fury: Inside the Trump White House," has also called for tougher libel laws.

"It's a disgrace that he can do something like this," said Trump, who has previously threatened to silence news organizations over critical coverage. "Libel laws are very weak in this country. If they were stronger, hopefully, you would not have something like that happen."

Trump's outburst magnified attention on the book that his aides have derided as "fantasy" and "complete fiction," but it also seemed to reveal a president who relishes constant conflict as feeling more besieged and isolated. With his approval ratings at historic lows after nearly one year in office, Trump has gone from battling Democrats and foreign leaders to fending off doubts from his closest advisers and even, reportedly, family members.

Wolff, appearing on NBC's "Today Show" on Friday, said that "100 percent" of Trump's team, including daughter Ivanka Trump and son-in-law Jared Kushner, both White House advisers, doubted the president's competency and grew more alarmed by his temperament during the first months of his presidency.

The controversy also served to elevate the question of Trump's fitness for office that some in Washington have privately - and some publicly - wondered about since before he took office.

During the campaign, Democrats, including President Barack Obama, called Trump "unfit" for the White House. Members of the Republican political and national security establishment called Trump dangerous to the nation, promoting a "never Trump" movement to try to undermine his candidacy.

"This isn't a situation where you have an episodic gaffe," Obama said in August 2016, after Trump engaged in a public dispute with the parents of a Muslim American soldier who died in combat in Iraq. "This is daily and weekly."

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Trump, 71, has boasted about his overall health even as his rivals have mocked his taste for fast food. In 2015, during his campaign, Trump released a statement from his longtime personal doctor, Harold Bornstein, that said he had "extraordinary" stamina and would be "the healthiest individual ever elected to the presidency." With his victory at 70, Trump became the oldest incoming president.

Even before his presidential campaign, Trump has defended his intelligence, repeatedly citing his high IQ.

"Sorry losers and haters, but my I.Q. is one of the highest -and you all know it! Please don't feel so stupid or insecure, it's not your fault," he wrote on Twitter in 2013.

Asked at Camp David why he felt compelled to defend himself over Wolff's book, Trump said: "Only because I went to the best colleges, or college." Trump, who spent two years at Fordham University before graduating from the Wharton School of the University of Pennsylvania, called himself a "very excellent student" who "came out and made billions of dollars as one of the top businesspeople and went to television and had many years of tremendous success."

He asserted that Wolff had not interviewed him, other than for a magazine story a "long time ago." But Wolff told NPR on Friday that he spent about three hours with Trump during the campaign and his presidency.

"I don't know this man," Trump said of Wolff. Referring to Bannon, whom the president has criticized for his casual dress code and disheveled appearance, Trump said: "I guess Sloppy Steve brought him into the White House a lot. I guess that's why Sloppy Steve is looking for a job."

The impetus for Trump's tweets appeared to be, as it often is, an early morning Fox News segment - this time about the controversy over Trump's mental health. White House aides were caught off guard. Chief of Staff John Kelly told reporters at Camp David that he had not seen Trump's morning tweets.

After a reporter showed him the messages, Kelly replied, "OK." The retired Marine general said Trump did not appear angry or agitated Friday night or Saturday morning and had tweeted to get around the "media filter" over coverage of Wolff's book.

Trump, for his part, praised reporters for having questioned some of Wolff's reporting tactics and the accuracy of passages in his book.

Trump's mental and emotional stability have become subjects of speculation at a time of great uncertainty in the country and the world. The president has engaged in a war of words with North Korean leader Kim Jong Un, boasting last week that he has a "much bigger & more powerful" nuclear button.

The tweet prompted reporters to raise the question of the president's mental fitness at recent White House press briefings. And last week, CNN asked Secretary of State Rex Tillerson, who last year denied reports that he had called the president "a moron."

"I've never questioned his mental fitness. I've had no reason to question his mental fitness," Tillerson said.

On Capitol Hill, the "nuclear button" tweet and a Politico report about a Yale University psychiatry professor recently briefing some lawmakers on Trump's behavior have sparked a new round of chatter. While concern about Trump is widespread in the Democratic Party, there are divisions about how to respond to him.