As Republican presidential nominee Donald Trump's verbal gaffes have mounted and his penchant for lashing out at his political opponents has continued to escalate, it's become in vogue for critics to earnestly raise questions about Trump's temperament and his mental stability.

Terms like "narcissist" and "sociopath" have been attached to his name as routinely as Trump attaches that name to his buildings. Many of Trump's GOP allies have been publicly apoplectic about his unorthodox (some have argued virtually non-existent) campaign, his contradictory statements regarding Russian President Vladimir Putin and his critical remarks about the Muslim-American parents of a slain Iraq War hero.

The Washington Post's Eugene Robinson even wrote a column this week asking: "Is Donald Trump just plain crazy?"

On MSNBC Tuesday, Joe Scarborough said he's heard similar questions from party insiders. "I fielded calls all day yesterday, from conservatives, from Republicans, from officials, people that the media would call right-wing bloggers ... and everybody was asking about his mental health," the "Morning Joe" host said. "It was all everybody was talking about yesterday ... everybody was calling me saying, 'What's happening to him?', 'What is wrong with him?'"

Added former Obama administration advisor and economic analyst Steven Rattner, "Somebody's gotta do a psychological profile of the guy and find out why he acts the way he acts, and is he really healthy."

Dr. Justin A. Frank, a Harvard-educated psychiatrist and author, knows a little something about the mental states of presidents. He's written two best-selling books, "Bush on the Couch" and "Obama on the Couch," which attempted to provide readers with insights into what made each leader tick.

Related: Trump Says He's 'Afraid' Election Is 'About to Be Rigged'

In Bush's case, Frank surmised that the 43rd president had never been properly treated for his alcoholism, and was a functioning "dry drunk" prone to making poor decisions and rationalizing them later. In Obama, Frank saw a man "who can be so present yet so absent," predisposed to seek consensus even when it was not to his advantage, in part because of Obama's complex childhood.

Frank has not delved deeply into Trump's psyche yet, and Frank cautions that he is not capable of diagnosing someone he has never treated. That said, he does find the real estate mogul fascinating and, at the very least, his public persona edifying.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

"What we see on TV seems very different from what we hear from friends of his," Frank told NBC News Monday. "He may be very different in a boardroom and on TV than the way his is at home. What we can say is that he does seem to believe in tit for tat. I think it's actually very self-protective."

Related: What Would It Take for the GOP to Rebuke Trump?

Frank sees Trump as a "brilliant" salesman and showman who has more in common with theatrical performers like Judy Garland or the fictional lead character in "All That Jazz" than anyone with a pathological disorder. So far it seems Trump's unpredictable instincts have served him well in business and in the 2016 campaign, and although Frank compares the GOP candidate's behavior "to somebody who is under 10 years old," it's actually Trump's followers who have Frank more unnerved.

According to Frank, Trump's repetition of phrases ("believe me") and pledges to single-handedly absolve the fears of the electorate have struck a chord, and have appealed to voters who crave reassurance regardless of the facts. Frank says this dissonance has inspired a "hot" internal debate within the psychiatric community about whether its appropriate to be more outspoken regarding troubling aspects of Trump's public persona.

"Most of my colleagues feel that he's not crazy, and if he is crazy, he's crazy like a fox," Frank said.

Dr. Henry A. Nasrallah, editor-in-chief of Current Psychiatry, wrote a tongue-in-cheek column in his publication last month lambasting "the least nuanced presidential campaign — ever" while calling out "an unabashed display of character flaws."

Nasrallah never mentions a specific candidate by name, but its hard not to deduce to which candidate he's referring when he suggests "the id has left the ego in its dust." Still, when asked to comment on his piece by NBC News, Nasrallah declined, and cited professional ethics.

Although he didn't name-check the term, Nasrallah was adhering to what has become commonly known as "The Goldwater Rule," an unofficial moratorium among the psychiatric community from weighing in on political candidates, which is supported by the American Psychiatric Association. It was inspired by the backlash to an unprecedented characterization of Sen. Barry Goldwater, the 1964 GOP nominee, by analysts at the time.

Goldwater's reactionary rhetoric during that race, and willingness to not rule out thermonuclear warfare, caused many voters at the time to question his mental health. The Arizona's senator's presumed instability was a significant pillar in Democratic President Lyndon Johnson's campaign against him, as evidenced by his infamous "Daisy ad."

That fall, with Goldwater's candidacy already faltering, the magazine Fact published two incendiary articles in a single edition (“The Unconscious of a Conservative: A Special Issue on the Mind of Barry Goldwater") inspired by a questionnaire sent to over 12,000 psychiatrists throughout the country that posed a loaded question: “Do you believe Barry Goldwater is psychologically fit to serve as president of the United States?”

Fallout from Fact's 1964 cover story helped create "The Goldwater Rule."

The vast majority of the recipients didn't respond (which Fact downplayed), but those who did (about 20 percent) overwhelmingly declared that Goldwater was unfit, and, in some cases used colorful language (words like "immature,” “impulsive,” “megalomaniac” — sound familiar?) that was excerpted to buttress their case. Fact hyped the 'consensus' of those who opposed Goldwater on its cover, feeding into a narrative that impartial professionals considered him unworthy. He would lose in a historic landslide that November.

The following year, a column in the American Journal of Psychiatry criticized the issue for its “political bias …wrapped up in pseudo-technical flagellation of Senator Goldwater." The senator sued Fact for libel and won, which led to the publication's demise, and the fallout led to the codification of "The Goldwater Rule," which most members of the psychiatric community have held to ever since.

Frank is old enough to remember the consternation around Goldwater's candidacy. Ironically, he remembers a friend who predicted that if Goldwater were elected president, the U.S. would "be at war within a year." That did occur, though under President Johnson. But as far as Frank is concerned, the hand-wringing over Trump is unlike anything he's even seen.

Though he's been what he describes as an "active Democrat" for many years, Frank actually admires Trump's willingness not to back down from his opponents' attacks.

"Too often Democrats have been passive. I was extremely bothered that John Kerry in 2004 didn’t immediately hit back when he was swift-boated," Frank said, referring to political attacks. "Trump would have hit back in the next minute. It's a very impressive quality he has, not to take anything lying down."

Related: Donald Trump Faces Temperament Question... Again

But, Frank cautions, "That might not be the right temperament to be president."

Perhaps Trump has become increasingly self-aware of that possibility. He has already started to float a conspiracy theory that the general election will be "rigged" against him, which some have interpreted as a way to cast doubt on a potential defeat. Others have speculated that Trump's entire campaign may simply be an elaborate launch for a new media empire. Scarborough suggested Tuesday that Trump doesn't actually want to govern, he simply wants to emerge victorious, a theory Frank finds plausible.

"The goal for him is to win. I would be surprised if he were thinking past victory," he said. "As far as saying something in public, you can talk about his impulse disorder, grandiosity, attention disorder — which is why he never reads — but you can’t make the diagnosis from observing him. I knew what we were going to get with Bush, I don’t know what we’re going to get with Trump. He needs to be evaluated."