Last year, I wrote an article calling Donald Trump a godsend for moderate Republicans. Trump, I predicted, would lose so spectacularly that the GOP would be forced to transform itself, surrendering its mindless obstructionism, science denial, xenophobia and plutocracy. After a purge like that, the party would finally be able to compete in future national elections.

I was wrong. I now see that Trump’s candidacy has exacerbated the Republican Party’s weaknesses, alienating minorities, fracturing the base and stunting smart policy development. The party’s structural problems are so sever...