Decades from now, historians will take a deep dive into the cesspool that was the 2016 presidential campaign.

As the future professors wallow through the mudslinging and misinformation of the Clinton-Trump conflict, they may look back at one defining moment when the election was ultimately decided.

It was one brief flash of clarity when Donald Trump was finally laid low by a previously unknown grieving father.

By now, nearly everyone has seen the emotionally charged video from the Democratic National Convention in Philadelphia when Khizr Khan criticized Trump's proposal to temporarily freeze the entry of foreign Muslims into the United States.

Khan, who is a Muslim, lost his son, Capt. Humayun Khan, in Iraq. Humayun Khan was awarded the Bronze Star and a Purple Heart for sacrificing himself to save his unit from a car bomb attack.

During his convention speech, Khizr Khan blasted Trump for his anti-Muslim policies before finally asking of the Republican candidate: “Have you ever even read the United States Constitution? I will gladly lend you my copy.” He then dramatically reached into his pocket for a small version of the historic document.

Not surprisingly, the Democratic crowd roared in approval.

He then told Trump: “You have sacrificed nothing and no one.”

It was a profoundly powerful statement.

Trump then responded by taking the low road, as usual. In the process, he took a bad situation and made it much, much worse.

He repeatedly bashed the Gold Star Khan family.

The backlash from veterans groups and even his Republican supporters was immediate and intense.

They wanted Trump to cease, desist and apologize.

Trump, however, doesn't do apologies. That's just not Donald's way, and he refused to retreat from his criticisms of the Khans.

In the past, none of Trump's outrageous mudslinging seemed to stick to “Teflon” Don. In fact, his supporters only cheered louder at every ugly insult and flat-out lie.

This time, however, was different. This time Trump ripped into the family of a war hero. This time Trump finally stepped well beyond the line.

His comments cost him almost immediately.

In the most recent polls, a race that had been nearly a dead heat suddenly found Hillary Clinton with a lead at or near double digits.

In the divided America in which we now live, 40 percent of the folks will always vote Republican and 40 percent will always vote Democratic. The battle is always for the 20 percent in the mushy middle.Trump's ill-advised reaction to the Khan family may have caused irreparable harm to his standing with that crucial constituency.

Of course, it's a painfully long election cycle. The vote is still three months away. A lot things can, and will, happen. Making a prediction at this juncture is perilous, at best.

Still, this has the feel of a true pivot point.

McCarthy comparison: This may indeed be a McCarthy Moment for Trump.

Joe McCarthy, for those who don't know their history, was a U.S. senator from Wisconsin from 1947 until 1957. He was made famous for his anti-communist bombast during the "Red Scare" in the 1950s. Like Trump, McCarthy had only an occasional relationship with the truth.

McCarthy made a career out of accusing innocent folks of being communists or communist sympathizers, all in an effort to raise his own political profile.

McCarthy didn't care who he hurt, as long as it increased his popularity.

The final straw for McCarthy came during a Senate hearing on June 9, 1954, when Boston lawyer Joseph Welch memorably asked him: “Have you no sense of decency, sir, at long last? Have you left no sense of decency?”

It was a seminal moment.

Finally, on Dec. 2, 1954, the Senate voted overwhelmingly (67-22) to censure McCarthy for his contemptible actions, making him one of the few senators to be disciplined in such a way.

McCarthy's political career was effectively over.

Trump may now face a similar fate.

In three months, the Republican candidate may face a landslide defeat to a significantly flawed candidate.

When the history of that potential defeat is written decades from now, the critical instant may have come on a hot summer night in the birthplace of our American republic, when an obscure but sorrow-stricken father unmasked the impostor who would be president.

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