President Donald Trump poses a significant threat of nuclear war and is increasingly dangerous, according to leading psychologists who are stepping forward to warn Americans of his escalatingly threatening behavior.

The Duty to Warn PAC, an organization of psychologists who believe Trump has a personality disorder called malignant narcissism, will host marches across the nation Saturday to warn people about a perceived increase in his volatility and unpredictability in his performance as president.

James Gillian, a licensed psychologist of 25 years, has spent his career working with mass murders, serial criminals and overseeing mental hospital programs for the most violent American criminals. He analyzed Trump’s statements for indications of danger for a The Dangerous Case of Donald Trump, a book with 27 expert psychiatrists analyzing the president’s behavior. 

Licensed psychologists will gather at a "Duty to Warn" event Saturday to precaution Americans about the mental state of President Donald Trump. Scott Olson/Getty Images

Gillian has analyzed quotes from Trump—spanning campaign calls for Hillary Clinton to be imprisoned or assassinated to more recent threats of nuclear action against North Korea. He said psychologists are bound by a "duty to warn," people about specific threats to safety, which sparked his co-authoring of the book. 

"I know more about the psychological side of violence than almost anyone in our society," Gillian told Newsweek. "When I say Trump is dangerous, the only question is, is that an understatement?"

John Gartner, a psychotherapist who taught psychiatric residents at Johns Hopkins University Medical School, founded the Duty to Warn PAC. The political organization plans to financially sponsor members of Congress who support legislation that invokes impeachment. Gartner said the PAC is not just focused on supporting Democrats. He told Newsweek, "If I can find a Republican doing it, I’ll give them double."

Gartner said licensed psychologists have used "concrete, observable traits" to determine that Donald Trump has a "pervasive pattern" of exploiting and violating the rights of others. The evaluations are based on Trump’s history, his statements and documented quotes of those close to him, Gartner said.

The boasting of sexual assault, allegedly not paying workers, the fraudulent Trump University case—Gartner said evidence points to malignant narcissism. The traits of malignant narcissism were coined by a German psychotherapist, Erich Fromm, who escaped Nazi Germany and developed the diagnosis to explain leaders who rose to power in World War II. 

"It was his attempt to explain Hitler, to explain Stalin and sadistic megalomaniacal dictators like them," Gartner told Newsweek. "When malignant narcissists gain power they become flagrantly worse."

The key patterns of behavior in that diagnosis: bragging, paranoia, conspiracy theories, demonization of those who oppose him, lying, exploitation and bullying. Gartner rejects the theory that Trump presents a public persona or uses the tactics for calculated political strategy.

"I’m an expert on personality disorders," Gartner said. "I don’t just know a media portrayal of Donald Trump. I have hundreds of hours of behavior that I have observed on video of his own words not mediated by anyone. I have more samples of behavior and speech from Donald Trump than most of my patients."

A long-held "Goldwater Rule" cast taboo on psychologists providing a diagnosis for public figures they never personally met. The principle emerged in the 1960s when psychiatrists were polled about whether politician Barry Goldwater was "unfit to be president." The results from psychologists were split, in reality, but a newspaper headline implied Goldwater was mentally unstable. Goldwater sued the newspaper and won.

Gillian believes that rule is misinterpreted.

"We’re not telling the public anything that Donald Trump hasn’t already told them," Gillian said. "We aren’t making this up. We’re listening to what he’s saying, hearing it and reminding the public."