There aren’t a lot of people who have been more anti-Trump than I have been for as long as I have been. I first wrote critically of Trump in 2011, because of his birther nonsense, and back in January I declared that I would not vote for him under any circumstances.

His actions since then have only confirmed my views and deepened my opposition to him. I've argued that he's the type of man the Founders feared.

I say all that to provide context in making this point: Republicans who say they will vote for Trump over Hillary Clinton are not necessarily or knowingly choosing party over principle or “selling their soul.” Some individuals may be—I’m not naïve; being a member of the human race I know motivations are complicated and easily defiled—but others are not. So it’s not fair to assume, absent fairly compelling evidence, that people are acting in cynical and unprincipled ways.

Take the conservative radio talk show host Dennis Prager as an example. Mr. Prager was a powerful and articulate Trump critic during the Republican primary. He made a very persuasive case for why voters should choose another, any other, Republican running for president over Trump. But he also said—and says today—that he would vote for Trump over Clinton.

The argument of people like Prager is that we know how Mrs. Clinton would govern if she were president: as a person of the left. In addition, she’s an ethical mess. The Trump-over-Clinton crowd also argue that Mrs. Clinton is sure to nominate Supreme Court justices that will lock in a liberal court for a generation. Trump may do that, too, but he may not. He might put an actual conservative on the Supreme Court. At least the chances of getting some good things done are better under a President Trump than a President Clinton.

I disagree with this bottom line judgment for several reasons. The first is that in considering those who run for the presidency, one needs to look beyond which candidate correctly checks the preferred policy boxes. That matters, but it’s not all that matters. And it may not even be what matters most.

Judgment, wisdom, temperament, and prudence are the most important qualities by which to evaluate a potential president. It’s obvious to me that Mr. Trump is not only temperamentally unsuited for the Oval Office; I think he’s quite dangerous—emotionally unstable, erratic, narcissistic, impulsive, cruel and vindictive. He is appealing to our darker impulses. He’s also stunningly uninformed and shallow, at least on matters of policy and philosophy. Even when running for president, he has shown no interest in even acquainting himself with the issues, let alone mastering them.

But there’s something else as well: Mr. Trump, if he were to win the presidency, would redefine the Republican Party and conservatism in ways that Hillary Clinton or Bernie Sanders never could. As president, he and the Republican Party would essentially become one. Trump and Trumpism would be definitional, and therefore discrediting. As Bret Stephens puts it:

Trumpism isn’t just a triumph of marketing or the excrescence of a personality cult. It is a regression to the conservatism of blood and soil, of ethnic polarization and bullying nationalism. Modern conservatives sought to bury this rubbish with a politics that strikes a balance between respect for tradition and faith in the dynamic and culture-shifting possibilities of open markets. When that balance collapses—under a Republican president, no less—it may never again be restored, at least in our lifetimes.

“The conservative movement can wait out a Clinton presidency intact,” added Jonah Goldberg. “A Trump presidency is a ride straight to perdition, with a capital H.”

That is where I have been and where I remain. Like other conservative commentators, I will continue to speak out against Trump during this campaign, despite the fact—and in some respects because of the fact—that he’s running as a Republican. It matters to me that he’s soiling the party of Lincoln and Reagan. I have higher expectations for my side than the other side.

I'll add that those who were supportive of Trump during the Republican primary have a great deal to answer for, when every other candidate running was (to me) so obviously superior to Trump. But Trump vs. Clinton is another, more complicated, matter. And while I believe I can make a persuasive case that voting for Donald Trump is wrong even if his opponent is Hillary Clinton, I don’t think all those who hold a different view than I do are intellectually unscrupulous or morally corrupt. We have a significant difference of opinion on the most important political issue of the day. But since my ability to apprehend the truth of things is at best partial, including on matters of politics, I’m not prepared to speak as if my judgment is the equivalent of having been written on stone tablets from Mt. Sinai.

All of which is to say I’m all in favor of a spirited and even an intense debate over the virtues and vices of Donald Trump. But people on both sides shouldn’t assume that those on the other side of the divide are unscrupulous or dishonorable.

They may simply be wrong.