The antidote to this threat, Lincoln argued, was to cultivate a “political religion” that emphasized “reverence for the laws.” Passion was our enemy, he warned; it had to be contained. “Reason — cold, calculating, unimpassioned reason — must furnish all the materials for our future support and defense.”

Lincoln was a keen student and great interpreter of the founders, and of course the founders also thought deeply about how a self-governing people could restrain political passions. In his book “Madison’s Metronome,” the scholar Greg Weiner points out that James Madison’s lifelong concern was that majorities would be governed by emotion rather than reason, the “cool” faculty. In Mr. Weiner’s words, Madison “portrayed passion through metaphors that suggested rapid and uncontrolled spread, including those of fires, fevers, pestilence and contagions.”

Before the Constitutional Convention, Madison undertook an extraordinarily thorough study of various forms of government. How might the Constitution protect us from what Aristotle called “the insolence of demagogues”?

Among the defects of ancient and modern republics, Madison wrote, were “popular assemblages, so quickly formed, so susceptible of contagious passions, so exposed to the misguidance of eloquent and ambitious leaders, and so apt to be tempted by the facility of forming interested majorities, into measures unjust and oppressive to the minor parties.”

Which brings us back to Donald Trump. No one would mistake Mr. Trump for eloquent, but he is a highly effective communicator in a political culture that is now almost indistinguishable from the reality TV culture from which he emerged. But the Trump phenomenon isn’t just about coarsening and stupidity: His political practices are precisely what the founders feared and Lincoln warned against.

When he was asked by CNN’s Jake Tapper about the sucker-punching episode, Mr. Trump responded by saying, “People come with tremendous passion and love for this country, and when they see protest — in some cases — you know, you’re mentioning one case, which I haven’t seen, I heard about it, which I don’t like. But when they see what’s going on in this country, they have anger that’s unbelievable. They have anger. They love this country.” In many respects, he added, “it’s a beautiful thing.”

This is an increasingly familiar refrain. When two brothers beat up a homeless Latino man last summer and cited Mr. Trump’s words as their justification — “Donald Trump was right, all these illegals need to be deported,” one of the men reportedly told the police — Mr. Trump responded by saying that while this was a shame, “I will say that people who are following me are very passionate.” His supporters, he said, “love this country and they want this country to be great again — they are passionate.”