But the reality is that Republican lawmakers have been put in this position because Mr. Trump was more popular among Republicans than anyone else in the race. He took the lead in the national polls in July 2015 and pretty much never lost it. Those of us who oppose Mr. Trump need to acknowledge that his European-style ethnic nationalism, which relies on stoking grievances, resentments and fear of the other — Mexicans, Muslims, Syrian refugees, the Chinese, etc.— has a powerful sway in the Republican Party today. To be clear, not all of Mr. Trump’s supporters are drawn to his ethnic nationalism. But all of his supporters are willing to accept it.

This is not the conservatism of William F. Buckley Jr. or Ronald Reagan or Jack Kemp; it is blood-and-soil conservatism primarily aimed at alienated white voters who believe they have lost the country they once knew. Trumpism also includes a heavy dose of conspiracy theories. It is no coincidence that Mr. Trump burst onto the national political scene in 2011 by claiming that Barack Obama, our first black president, was not a natural-born American citizen but rather was born in Kenya…

The stain of Trump will last long after his campaign. His insults, cruelty and bigotry will sear themselves into the memory of Americans for a long time to come, especially those who are the targets of his invective.

Mr. Trump is what he is — a malicious, malignant figure on the American political landscape. But Republican primary voters, in selecting him to represent their party, and Republican leaders now rallying to his side, have made his moral offenses their own.