That a demagogue like Mr. Trump would emerge when there is a lot frustration in the lives of Americans who feel that their suffering is ignored isn’t without precedent. Which is why the greater sin of the Republican Party wasn’t that Mr. Trump won the nomination by carrying a plurality of votes in a large field. It was that people who surely knew better rallied to Mr. Trump once he became the nominee. Some advised him, others defended him and excused him, and still others tried to ignore him. Certain people acted worse than others. But in the end, they were all caught up in Mr. Trump’s ethical confusion and moral corruption. They didn’t pull him up; he pulled them down.

It is not as if the trends cited above were unknown to responsible Republicans and conservatives before the advent of Mr. Trump. They were, and some party loyalists challenged them at the time. Those efforts clearly failed, and Republicans have to come to terms with the fact that the rot was far more advanced than we understood.

Those of us who have long defended the Republican Party could do worse than honestly assess what role we might have played in all this, what we missed and why, and what more we could have done to stop it. When you are part of a political movement that chose Donald Trump as your nominee, some serious self-reflection is in order.

For now, though, the Republican Party faces a crisis. If Mr. Trump loses, the party faces a daunting reconstruction challenge. Policies that promote economic growth, social mobility and greater opportunity are important. But in some respects the party’s stance on the tax code, wage subsidies, higher education, tax credits or entitlement programs are a secondary priority.

Republicans need to wrestle with more fundamental questions first: Will their party choose as its leaders people who respect democratic institutions and traditions, or not; who conceive of America as a welcoming society or as one that is racially and religiously closed; who are committed to helping or exploiting the weak and vulnerable; who admire or oppose tyrants; who respect truth or view it in purely utilitarian ways; who abhor ignorance or embrace it? Will Republicans gravitate toward leaders who have authoritarian tendencies, who incite violence in their followers, and whose personalities are vindictive, cruel and disordered?

In a post-Trump world, Republicans need to ask themselves if their party will be characterized by its aspirations or its resentments. Can it make its own inner peace with living in an increasingly diverse and nonwhite America? Does it conceive of its role as tamping down or inflaming ugly passions? Does it believe in a just social order or not?

These questions are not about whether the economic concerns of Mr. Trump’s core constituency should be taken seriously and addressed by Republican policy makers. They clearly should, and in fact many of the people who urged the party to focus on this long before Mr. Trump entered the race are his most scathing critics. These questions go to how the Republican Party conceives of itself, its role and purpose in our political system and our common life. No single person can answer them. This self-definition rests with Republicans in every state and social stratum.