Story highlights Last week, Khizr Khan, a Harvard-trained Gold Star father, said that Donald Trump did not understand the meaning of true sacrifice.

Charles Kaiser: At a pivotal moment in American history, Khan's comment may be the reason it also becomes a righteous one.

Charles Kaiser is the author of "1968 in America," "The Gay Metropolis" and, most recently, "The Cost of Courage." The opinions expressed in this commentary are his.

(CNN) Sixty-two years ago, Joseph Welch interrupted Wisconsin Sen. Joe McCarthy when he was in the middle of smearing Fred Fisher, a lawyer in Welch's office. Welch stopped the communist-hunting McCarthy in his tracks on national television.

"Until this moment, senator, I think I have never really gauged your cruelty or your recklessness," said Welch. "Have you no sense of decency, sir? At long last, have you left no sense of decency?"

Charles Kaiser

This was the turning point in the hysteria that McCarthy had whipped up. With half a dozen sentences, the rumpled Harvard lawyer had pierced the McCarthy bubble of fear, which had paralyzed the country for years.

It is possible that when Americans look back 50 years from now at the present ghastly moment, it will be another speech by another middle-aged Harvard lawyer -- this one named Khizr Khan -- that will be remembered as the turning point in the national hysteria over Trump. Khan is the father of a genuine American war hero, and it fell to him to deliver the speech at the Democratic National Convention, which captured genuine American values more beautifully than anyone else could:

"Have you ever been to Arlington Cemetery?" Khan asked Trump. "Go look at the graves of the brave patriots who died defending America -- you will see all faiths, genders and ethnicities. You have sacrificed nothing and no one."