Once in a while, you can tell that a political moment will be burned into your memory forever, etched there like a stone engraving until the day you die.

For me, one of them came Thursday night when Khizr Khan reached into his blazer on the stage at the Democratic convention and pulled out his frayed copy of the United States Constitution.

"Donald Trump, you're asking Americans to trust you with their future," he said. "Let me ask you: Have you ever read the United States Constitution?"

Then, waving his miniature pocket edition, he delivered the killer line: "I will gladly lend you my copy!"

It was the emotional peak of the convention, and it sent the crowd jumping to its feet.

But this was not theater. Khan spoke with the solemn conviction of a man who sacrificed his son for a cause that he considers sacred, the defense of America.

It happened in Iraq in June 2004, more than a decade ago. But as anyone who has lost a child can tell you, it might just as well have been last week. These wounds don't heal.

His son, Army Capt. Humayun Khan, commanded a squad of soldiers protecting the perimeter of a base in Baquba. One afternoon, an orange taxi approached the gate, arousing suspicions. Khan ordered his men to hit the dirt.

He took the risk on himself. He took 10 steps toward the car, and it exploded, killing him in an instant.

His mother, Ghazala Khan, had last spoken to him a month earlier, on Mother's Day. "Whenever I talked to him, I started to cry," she told the Washington Post. "He always said to me, 'Don't worry. I'm safe.' "

His father later wondered if things could have turned out differently, somehow.

"Where did his strength come from to face such a danger instead of hiding behind a pole or booth or something?" he asked. "Normally we would try to hide. Had he done that, there would be no problem at all."

This sad story of loss is not unique. A brave young man protecting his crew with his life. A mother sick to her bones with worry. A wounded father, bursting with both pride and regret.

American families have walked this path thousands of times, after Bunker Hill, after Gettysburg, after Fallujah. The Khan family is now part of a noble brotherhood, one that is about patriotism and sacrifice and community.

What it is not about is religion. The Khans are Muslim, which makes absolutely zero difference.

That is what Trump fails to understand. And that is why Khan spoke with such contained fury on that stage. His son died for a cause, and Trump has been chipping away at it to score cheap political points.

Read the Constitution, he demanded of Trump. You'll see that it is about common people joining together to establish what the preamble describes as "a more perfect union."

It is not about chopping America into pieces, about setting one group against another by stirring fears, by blowing on coals to inflame bigotry, by inciting violence as a means of political expression.

In Philadelphia last week, Trump took a series of body blows that were well-deserved. He was slammed for stiffing small-business owners in Atlantic City, not paying money he had promised in signed contracts; for telling Americans he was ready to be our ruler and only he knew how; for abandoning allies that had sacrificed lives for us; for his pathological ego and lack of humility. All that is undeniable, and should be enough to disqualify him.

But Khan may have landed the hardest blow of all: "You have sacrificed nothing, and no one."

In Cleveland the week before, Trump's life story was presented as a triumph that grew from his business genius, which he was ready to put at the service of the nation.

The speeches didn't mention that he was born into wealth, that he avoided service in Vietnam with student and medical deferments, or that he dismissed Sen. John McCain's heroism by saying, "I like people who weren't captured."

Anyone who can speak like that about McCain doesn't understand patriotism, or the common cause that undergirds it.

By pointing to that with such moral authority, Khan exposed the corrosive nature of Trump's dark brand of politics. If Trump can win this election by setting Americans against one another, does anyone doubt by now that he would jump at the chance?

I fear that Trump could do lasting damage, unless he is crushed in November. Because he's shown there is a market for demagoguery, that the optimistic and unifying appeals made by leaders like Ronald Reagan and Barack Obama are just one option.

Gov. Chris Christie sees it. He's running around New Jersey suburbs now peddling his plan to take billions of dollars from school systems in poor cities and shift it to the suburbs. It's a poisonous plan that would cut spending in Newark and Camden by more than half, and Christie so far has refused to even hold a public hearing in a city.

But in the suburbs, he's telling voters they are "getting shafted every year, over and over again."

He didn't talk like that before Trump arrived. But to Christie, as with Trump, it's all about winning. And if tearing the fabric of this state does the job, so be it.

Capt. Khan's family is of Pakistani descent, and came to America from the United Arab Emirates when he was 2. His family is just the type that would be barred from America under Trump's ugly nativist plan.

He was buried in Arlington Cemetery, honored in a military ceremony as his coffin, draped in American flag, was carried to his grave site.

"If it was up to Donald Trump, he never would have been in America," his father said Thursday.

Maybe then Khan's comrades in Iraq would have died instead. Maybe not.

But one thing is certain: Trump somehow has missed one of the core ideas that make this country so great, that we are far richer with families like the Khans aboard, that the content of their character is what counts, not their religion.

If he wanted to understand that, he could start by taking up Khan's suggestion.

More: Tom Moran columns

Tom Moran may be reached at tmoran@starledger.com or call (973) 836-4909. Follow him on Twitter @tomamoran. Find NJ.com Opinion on Facebook.