Robert Kagan is a senior fellow at the Brookings Institution and a contributing columnist for The Post.

One wonders if Republican leaders have begun to realize that they may have hitched their fate and the fate of their party to a man with a disordered personality. We can leave it to the professionals to determine exactly what to call it. Suffice to say that Donald Trump’s response to the assorted speakers at the Democratic National Convention has not been rational.

Why denigrate the parents of a soldier who died serving his country in Iraq? And why keep it going for four days? Why assail the record of a decorated general who commanded U.S. forces in Afghanistan? Why make fun of the stature of a popular former mayor of New York? Surely Trump must know that at any convention, including his own, people get up and criticize the opposition party’s nominee. They get their shots in, just as your party got its shots in. And then you move on to the next phase of the campaign. You don’t take a crack at every single person who criticized you. And you especially don’t pick fights that you can’t possibly win, such as against a grieving Gold Star mother or a general. It’s simply not in your interest to do so.

The fact that Trump could not help himself, that he clearly did, as he said, want to “hit” everyone who spoke against him at the Democratic convention, suggests that there really is something wrong with the man. It is not just that he is incapable of empathy. It is not just that he feels he must respond to every criticism he receives by attacking and denigrating the critic, no matter how small or inconsequential the criticism. If you are a Republican, the real problem, and the thing that ought to keep you up nights as we head into the final 100 days of this campaign, is that the man cannot control himself. He cannot hold back even when it is manifestly in his interest to do so. What’s more, his psychological pathologies are ultimately self-destructive. (Disclosure: I was a guest speaker at a fundraiser for Democratic presidential nominee Hillary Clinton last month; I have no role with her campaign.)

[Robert Kagan: This is how fascism comes to America]

Both Republicans and Democrats are publicly responding to the row between GOP presidential nominee Donald Trump and Khizr Khan. Khan's son was killed while serving in Iraq. Here's what politicians from both parties said. (Monica Akhtar/The Washington Post)

Trump is, in this respect, unlike a normal politician. A normal politician knows that no matter how much criticism gets under the skin, the thing to do is to smile and wave it off. You don’t have to mean it. You don’t even have to appear to mean it. But it is what you do, if only to avoid compounding the damage. Trump cannot make this simple self-serving calculation. He must attack everyone who opposes him, even after he has defeated them. He must continue talking about Texas Sen. Ted Cruz’s father, even after Cruz has thrown in the towel. He must humiliate New Jersey Gov. Chris Christie, even after Christie has lain down before him.

Many of Trump’s supporters admire him for his bold challenge to political correctness. But his political incorrectness may be only an unintended side effect of his malady. Some of the insults he fires back at his critics are politically incorrect: the racist and misogynist taunts. But others are just childish: making fun of someone’s height, or suggesting that someone’s father was involved in the Kennedy assassination. It’s not really politically incorrect to say that a prisoner of war is not a hero because he got captured. It’s just a way of saying, I don’t care if you’re a war hero. You criticized me and now I’ve got to hit you. Trump’s insults are scattershot — only sometimes touching the raw racist and xenophobic nerves in society. The most important fact is that he is unable to control his responses to criticism. He must double down every time, even if it means digging himself deeper and deeper into the hole.

Imagine such a person as president. What we have seen in the Trump campaign is not only a clever method of stirring up the anger in people. It is also a personality defect that has had the effect of stirring up anger. And because it is a defect and not a tactic, it would continue to affect Trump’s behavior in the White House. It would determine how he dealt with other nations. It would determine how he dealt with critics at home. It would determine how he governed, how he executed the laws, how he instructed the law-enforcement and intelligence agencies under his command, how he dealt with the press, how he dealt with the opposition party and how he handled dissent within his own party. His personality defect would be the dominating factor in his presidency, just as it has been the dominating factor in his campaign. His ultimately self-destructive tendencies would play out on the biggest stage in the world, with consequences at home and abroad that one can barely begin to imagine. It would make him the closest thing the United States has ever had to a dictator, but a dictator with a dangerously unstable temperament that neither he nor anyone else can control.

One can hope it does not come to that. In all likelihood, his defects will destroy him before he reaches the White House. He will bring himself down, and he will bring the Republican Party and its leaders down with him. This would be a tragedy were it not that the party and its leaders, who chose him as their nominee and who now cover and shill for this troubled man, so richly deserve their fate.