Obviously, our competing interpretations of Mr. Trump can be traced to the fact that we view him in fundamentally different ways. But because we’ve shared a similar outlook in the past, this divergence is particularly frustrating for my friends now.

In political debates we assume wisdom resides with us and not our opponents. There’s nothing intrinsically wrong with that; it’s the reason we hold the views we do. And so when I see so many Republicans defend Mr. Trump regardless of his actions — invoking defenses that I am certain would enrage them if champions of a President Hillary Clinton had said the same things on her behalf — I’m convinced we’re seeing a severe case of confirmation bias, the tendency to interpret new evidence as confirmation of one’s existing beliefs.

But here’s the thing: What’s easy to see in others is hard to see in ourselves. I can assure myself that my intellectual integrity is superior to theirs, yet in my honest moments I recognize that I struggle with these same human frailties and flaws.

I have some of the same mental habits that I’m critical of in others.

I know it’s a struggle for me to see Mr. Trump, whom I consider to be malicious, in a disinterested way. I know, too, that I’m quick — most Republicans would say much too quick — to home in on his failures, to focus on the things he does that confirm my concerns about him. That doesn’t necessarily mean, of course, that my judgments about Mr. Trump aren’t in the main correct. I believe they are. History will sort out whose judgments were vindicated and whose were not. I’m simply saying that for me to see Mr. Trump from a distance, dispassionately, is impossible. So my views of him, even if they are basically accurate, are also incomplete and probably distorted.

Another example: the Iraq war. When I served in the George W. Bush White House, I believed before the war began that it was justified — that Saddam Hussein possessed weapons of mass destruction, that he was a particularly malevolent and destabilizing figure, and that it was a military conflict that would liberate an enslaved people.