For the GOP, this has all been a godsend, an opportunity to amplify the case being made every day by the Democrats themselves against their own stewardship. Instead, the Republicans spent the summer attacking each other — the festival of ad hominems interrupted only by spectacular attempts to alienate major parts of the citizenry.

The latest example is Ben Carson, the mild-mannered, highly personable neurosurgeon and one of two highest-polling GOP candidates. He said on Sunday that a Muslim should not be president of the United States.

His reason is that Islam is incompatible with the Constitution. On the contrary. Carson is incompatible with a Constitution that explicitly commands that “no religious test shall ever be required as a qualification to any office or public trust under the United States.”

Ever. And it is no defense of Carson to say that he was not calling for legal disqualification of Muslims, just advocating that one should not vote for them. That defense misses the point: The Constitution is not just a legal document. It is a didactic one. It doesn’t just set limits to power; it expresses a national ethos. It doesn’t just tell you what you’re not allowed to do; it also suggests what you shouldn’t want to do. For example, the First Amendment allows you to express whatever opinion you want — even, say, advocating the suppression of free speech in others. But a major purpose of the Constitution is to discourage and delegitimize such authoritarian thinking.