In some ways, it was the most memorable line of Monday’s presidential debate.

It came after Donald Trump had stumbled through a non-responsive response to a simple question: Why did it take him so long to concede President Obama was born in the USA? In reply, Trump congratulated himself for getting Obama to produce his long-form birth certificate, falsely blamed Hillary Clinton for starting the racist campaign and touted his great relationship with black people.

And Clinton, smiling, said, “Just listen to what you heard.”

They ought to put that on a billboard; it could encompass Trump’s whole campaign. It certainly encompasses his debate performance. Granted, Trump spent the first minutes doing his best impression of a statesman, but that was all he could manage. Then, like your favorite band on a reunion tour, he pulled out the old hits.

He blustered, filibustered and interrupted, insisted (dishonestly) that he opposed the Iraq War from the beginning, suggested (deceptively) that he never said Clinton doesn’t have “a presidential look,” declared (mendaciously) that he never called climate change a Chinese “hoax.”

At one point, he answered a question about cybersecurity by noting that his 10-year-old son “is so good with these computers, it’s unbelievable.” Which is lovely for little Barron Trump, but tells us nothing about how his dad would secure the American computer grid against state-sponsored hackers.

Perhaps most incredibly, at the end of a testy rant, the notoriously thin-skinned entertainer said, “I … have a much better temperament than she has.” Reality itself lurched sideways on that one.

“Just listen to what you heard,” she said, i.e., take it at face value. If we were all doing that, this election wouldn’t be close. But we aren’t, so it is.

It’s important to understand that, at its core, Trump’s appeal is neither about issues nor policy positions. Nor is it primarily, as some would argue, grounded in economic dislocation. Yes, the Rust Belt is hurting. But it’s been hurting for years; the era when a high school education got you a lifetime job on the loading dock or factory floor has been gone for a long time.

By contrast, the era of political incoherence that has produced Trump is a relatively new phenomenon. So perhaps the dislocation we’re talking about is less economic than demographic, i.e., the rancor of those who resent pressing one for English, transgender bathrooms, two men atop a wedding cake and a brown-skinned president with a funny name singing Al Green at a campaign rally. Some of us feel steamrolled by change.

They say they want to “make America great again.” For them, America was “great” when they were the only ones who had a say in it.

Abraham Lincoln famously said that no foreign power could, by force, “take a drink from the Ohio or make a track on the Blue Ridge, in a trial of a thousand years. … If destruction be our lot, we must ourselves be its author and finisher.” This was 23 years before the Civil War. Almost 200 years later, we face another civil war, only it’s a civil war of ideas and ideals, a secession from objective reality and the greater us.

But Lincoln is still right — if destruction is our fate, it will come from within. And with the arguable exception of the 2008 recession, Donald Trump represents this country’s gravest existential threat since the end of the Soviet Union.

“Just listen to what you heard,” advises Hillary Clinton.

And it’s a marker of what we’ve become that for some of us, that will be entirely too much to ask.

Leonard Pitts Jr. is a columnist for the Miami Herald. Readers may write to him via email at: lpitts@miamiherald.com.