“I think you’d have riots.” So said Donald J. Trump last week, when he was asked by CNN what he thought would happen if he arrived at the Republican Convention this summer a few delegates short of the 1,237 needed to win outright and didn’t set forth from Cleveland as the party’s nominee.

It is stunning to contemplate, particularly for those of us who are lifelong Republicans, but we now live in a time when the organizing principle that runs through the campaign of the Republican Party’s likely nominee isn’t adherence to a political philosophy — Mr. Trump has no discernible political philosophy — but an encouragement to political violence.

Mr. Trump’s supporters will dismiss this as hyperbole, but it is the only reasonable conclusion that his vivid, undisguised words allow for. As the examples pile up, we should not become inured to them. “I’d like to punch him in the face,” Mr. Trump said about a protester in Nevada. (“In the old days,” Mr. Trump fondly recalled, protesters would be “carried out in a stretcher.”)

Of another protester, Mr. Trump said, “Maybe he should have been roughed up.” In St. Louis, Mr. Trump sounded almost wistful: “Nobody wants to hurt each other anymore.” About protesters in general, he said: “There used to be consequences. There are none anymore. These people are so bad for our country. You have no idea folks, you have no idea.”

Talk like this eventually finds its way into action. And so on March 10, a Trump supporter named John McGraw, was charged with assault, battery and disorderly conduct, after a protester was sucker-punched as he was being hauled by security guards out of a Trump rally in North Carolina the day before. When interviewed afterward Mr. McGraw said, “The next time we see him, we might have to kill him.”

And Donald Trump’s reaction? He said he was considering paying Mr. McGraw’s legal fees. “He obviously loves his country,” Mr. Trump added, “and maybe he doesn’t like seeing what’s happening to the country.”

Welcome to Donald Trump’s America.

Mr. Trump’s comments, startling in a leading presidential candidate, have raised widespread concern about the path we find ourselves on. But concern about political violence, mob rule and unchecked passion is hardly new in American history.

In 1838, as a 28-year-old state legislator, Abraham Lincoln delivered an address at the Young Men’s Lyceum of Springfield, Ill. The speech was given in the aftermath of the lynching of a mixed-race boatman and the killing of an abolitionist newspaper editor. Lincoln warned that a “mobocratic spirit” and “wild and furious passions” posed a threat to republican institutions. He also alerted people to the danger of individuals — “an Alexander, a Caesar or a Napoleon?” — who, in their search for glory and power, might pose a threat to American self-government.

“Is it unreasonable, then, to expect that some man possessed of the loftiest genius, coupled with ambition sufficient to push it to its utmost stretch, will at some time spring up among us?” Lincoln asked.

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.”

Note Mr. Trump’s linkage of violence, passion, anger and love of country. After the sucker-punch, Mr. Trump, while himself protesting that he doesn’t condone violence, initially indicated that he might subsidize it. He said that he hoped that he hadn’t done anything to create a tone where violence was encouraged, even though he does just that. Last week, his campaign manager, Corey Lewandowski, was accused of manhandling a reporter and then sought to discredit her on Twitter and elsewhere. Mr. Trump went out of his way to praise Mr. Lewandowski during his victory speech in Florida.

For Mr. Trump, this is all of a piece. His entire campaign, from its very first moments, has been built on stoking anger, grievances and resentment against people of other races, religions and nationalities. Mexicans coming to America are rapists and drug dealers. Muslims hate America and need to be barred from it. Syrian refugees are “Trojan horses.”

Not surprisingly, Mr. Trump’s politics of hate is now metastasizing into violence. He incites people — not all people to be sure, but enough. On social media in particular, one sees how he gives his supporters permission to express dark and ugly sentiments that existed before but were generally kept hidden from view.

Max Boot, a Republican Trump critic who was a foreign policy adviser to Marco Rubio’s campaign, says that he has never experienced as much anti-Semitism as he has since the start of the Trump campaign. There are no filters anymore, no restraints, no cultural guardrails. Now, under the sway of Trumpism, what was once considered shameful asserts itself openly. As we contemplate this, it is worth recalling that the membrane separating what the Scottish novelist John Buchan called “the graces of civilization” from ”the rawness of barbarism” is thinner and more fragile than we sometimes imagine.

The reasons for the rise of Mr. Trump are undoubtedly complicated and will be studied for decades to come. That Mr. Trump’s rise has occurred in the Republican Party is painful for those of us who are Republicans. That more and more Republicans are making their own accommodation with or offering outright support for Mr. Trump — governors like Chris Christie and Rick Scott, the former candidate Ben Carson and the former speaker of the House Newt Gingrich — makes things even worse. Because we can no longer deny what Mr. Trump is and what he represents. The prospect of turning the party apparatus over to such a person is sickening.

The founders, knowing history and human nature, took great care to devise a system that would prevent demagogues and those with authoritarian tendencies from rising up in America. That system has been extraordinarily successful. We have never before faced the prospect of a political strongman becoming president.

Until now.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center, served in the last three Republican administrations and is a contributing opinion writer.