This is the excellent advice of Alan Dershowitz (emphasis added):

But psychiatrists and other mental health professionals have no more right to pathologize a president or a candidate because they disagree with his or her political views than do prosecutors or politicians have a right to criminalize political opponents.

I have been writing in opposition to the criminalization of political differences for decades, because it is dangerous to democracy. It is even more dangerous to pathologize or psychiatrize one’s political opponents based on opposition to their politics.

Getting mental health professionals to declare political opponents mentally ill was a common tactic used against political dissidents by the Soviet Union, China, and apartheid South Africa. Perfectly sane people were locked up in psychiatric wards or prisons for years because of phony diagnoses of mental illness.

The American Psychiatric Association took a strong stand against the use of this weapon by tyrants. I was deeply involved in that condemnation, because I understood how dangerous it is to diagnose political opponents instead of responding to the merits of their political views.

It is even more dangerous when a democracy like the U.S. begins to go down the road of pathologizing political differences. It’s one thing to say your opponents are wrong. It’s quite another to say they are crazy.

Questions about President Trump’s mental health arose even before he was elected. Throughout the 2016 presidential campaign, some of his most extreme critics were not content to say they disagreed with his policies – or thought he was unqualified because of his temperament, background, or skill set. Instead, they questioned his mental health.

I am old enough to remember the last time this happened. The 1964 presidential election was the second in which I voted. President Lyndon Johnson, who had succeeded the assassinated President John F. Kennedy, was running against Sen. Barry Goldwater, R-Ariz.

I didn’t like either candidate. Johnson’s personal characteristics were obnoxious, though he had achieved much, especially in the area of civil rights. Goldwater’s personal characteristics seemed fine, but I disapproved of his conservative political views.

I was shocked to read an article in Fact magazine – based on interviews with more than 1,100 psychiatrists – that concluded Goldwater was mentally unstable and psychologically unfit to be president. It was Lyndon Johnson whose personal fitness to hold the highest office I had questioned.

Goldwater seemed to me to be emotionally stable, with excellent personal characteristics, but highly questionable politics. The article was utterly unpersuasive, but in the end, I reluctantly voted for Johnson because Goldwater was too conservative for my political tastes.

Goldwater went back to the Senate, where he served with great distinction and high personal morality. Johnson got us deeply into an unwinnable war in Vietnam that hurt our nation and claimed more than 58,000 American lives. The more than 1,100 psychiatrists, it turned out, were wrong in their diagnosis and predictions.

The misdiagnosis of Goldwater should surprise no one, since none of the psychiatrists had ever examined, or even met, the Arizona senator. They just didn’t like his politics. Indeed, some feared that he would destroy the world if he had access to the nuclear button.

The most powerful TV ad against Goldwater showed an adorable young girl playing with a flower. Then, the viewer hears an ominous voice counting down from 10, the camera zooms into a tight close-up of the little girl’s eye, and you see the horrific mushroom cloud of a nuclear explosion, implying that electing Goldwater would bring about a nuclear holocaust. It was an effective ad. It influenced me far more than the psychobabble in the Fact article.

Read it all.

I would add that those who suffer from Trump Derangement Syndrome are in no position to call Trump crazy or mentally unstable.  That would be a case of the pot calling the kettle black.

But haven't I just done what Dershowitz says one ought not do? Have I not just pathologized the views of those who oppose Trump by calling these people deranged?

No. I am not pathologizing their views, I am pathologizing them in respect of their boundless hatred of the man. Robert de Niro is a prime example. In his latest outburst, he calls Trump in public a "fucking idiot" and a "fucking fool" and on and on.  And there is this even worse earlier stream of invective from De Niro.  

I call this phenomenon topical insanity. There are certain topics that will 'trigger' ordinarily sane people and cause them to lose their mental stability.  Guns have quite the triggering effect on many liberals.  They simply cannot maintain their mental balance when the topic comes up. Pointing out well-known truths about race will do it as well.

So we need to distinguish between pathologizing views and pathologizing people.

There are a number of interesting questions here.  One question is whether there are any political or other views which are such that their holding by anybody would be good evidence of mental instability on the part of the one holding the view. 

A related but different question is whether there are any political or other views which are such that their holding by anybody would be good evidence of moral corruption or an evil nature.

Finally, there is the phenomenon of calling one's political opponents stupid. This is obviously different from calling them either insane or evil.  For example, I have heard Ann Coulter called stupid. But stupid is one thing she is obviously not.  Every political view has adherents that are less and more intelligent.  For example, Nancy Pelosi is not very bright as should be obvious. Obama on the other hand, is quite bright and indeed brighter, I would judge, than Joe Biden or  G. W. Bush.  But having a high degree of verbal intelligence is no guarantee that one possesses wisdom or has the right values.