Democrats can resort to this sort of sniping, too. Many Trump critics in 2016, and in the year after his election, pushed the idea that his father had suffered from Alzheimer’s and now Trump was losing it and that his vocabulary was eroding.

And it has become common among his attackers to say the president is deranged, suffering from malignant narcissism.

In his new book, “Front Row at the Trump Show,” Jonathan Karl, the chief White House correspondent for ABC News, reports the surprising fact that one of those calls on Trump derangement came from inside the White House.

Karl recounts that when Mick Mulvaney became acting chief of staff, he took senior White House staffers to Camp David for a weekend retreat. He recommended they read a 2011 book, “A First-Rate Madness: Uncovering the Links Between Leadership and Mental Illness,” by Nassir Ghaemi, director of the mood disorders program at Tufts Medical Center.

“This book argues that in at least one vitally important circumstance insanity produces good results and sanity is a problem,” Ghaemi writes in his introduction. “In times of crisis, we are better off being led by mentally ill leaders than by mentally normal ones.”

Lincoln and Churchill had bouts with depression and Gen. William Sherman suffered from paranoid delusions during the Civil War. But less mercurial leaders like Neville Chamberlain and Gen. George McClellan fared badly.

As Karl writes: “The new acting chief of staff seemed to be saying President Trump was mentally ill — and that this was a good thing. The corollary to that theory: Don’t try to control the man in the Oval Office. What you think is madness is actually genius.”