Mahalia Jackson sings at the March on Washington for Freedom and Jobs in 1963 (photo: Bob Parent Collection/Hulton Archive)

Martin Luther King Jr. is widely and wisely credited for giving one of the most galvanizing speeches in recorded history with his “I Have a Dream” address of Aug. 28, 1963. But he might never have “dreamed” at all on that occasion almost 53 years ago if not for some goosing from gospel legend Mahalia Jackson.

Jackson was on hand to sing at the Lincoln Memorial before and after King addressed the March on Washington for Jobs and Freedom. And if she’d only climaxed the civil rights event by performing “How I Got Over,” that would have been history enough. But her greatest contribution to the occasion went unheard by the vast crowd on the Washington mall, and it consisted of one single exhortation:

“Tell them about the dream, Martin! Tell them about the dream!”

That’s what witnesses close to the podium heard the gospel great shout midway through the speech. And it seemingly had a startling effect. With nary a pause, King pushed the notes from which he’d been reading off to the side. He no longer is seen looking down at the prepared text, and as he goes extemporaneous, King’s calm voice suddenly takes on a preacherly oratorical style. He is winging it. He is telling them about the dream.

And by “them,” we mean not just the tens of thousands on hand in D.C., but the tens of millions who would hear recorded snippets of this spontaneity over the next five decades.

It was “one of the world’s greatest gospel singers shouting out to one of the world’s greatest Baptist preachers,” speech writer Clarence Jones told the New Orleans Times-Picayune. “She may have ignored the fact that there were almost 300,000 other people there, and she just shouted out to Martin, ‘Tell them about the dream.’ Anybody else who would yell at him, he probably would’ve ignored it. He didn’t ignore Mahalia Jackson. I said to somebody standing next to me, ‘These people don’t know it, but they’re about ready to go to church.’”

Jones had collaborated with King on the original version of the speech, but he was hardly offended when the civil rights leader suddenly deviated from the text. “When Mahalia said that, it was almost like a mandate to respond,” he told a Television Critics Association panel in August 2013, while promoting The March, a PBS special narrated by Denzel Washington. “I could see his body language change from the rear. Where he had been reading, like giving a lecture, but then going into his Baptist preacher mode.”

On another occasion that year, Jones said, “I have never seen him speak the way I saw him on that day. It was as if some cosmic transcendental force came down and occupied his body. It was the same body, the same voice; but the voice had something I had never heard before. It was so powerful, it was spellbinding.”

Mahalia Jackson was the wind beneath King’s wings on more than one occasion. “When he was down — or the classic word that’s thrown around today, that word ‘depressed’ — he would ask his secretary Dora McDonald, he would say, ‘Dora, get Mahalia on the phone,’” Jones told the Times-Picayune. “And he would say, ‘Mahalia, I’m having a rough day. Sing for me.’ And Mahalia would sing to him in the phone. He would say, ‘Sing, “Jesus Met the Woman at the Well” or “The Old Rugged Cross,”’ or other favorites. And he would listen to her voice through the phone, and sometimes tears would come down his face.”

It wasn’t as if Jackson was aware of King’s REM sleep patterns. “I have often speculated that she had heard him talk in other places, and make reference to the dream,” Jones told the TCA panel. For example: “On June 23, 1963, in Detroit, he had made very express reference to the dream.”

What was the dream, anyway? A combination of gospel-rooted idealism and hard-nosed political pragmatism. Writer Joseph E. Davis pointed out that the speech was originally intended to be lower-key, since King wanted to appeal to a vast audience, and knew he was dealing with then-sensitive themes. “It is always overlooked that King bookended ‘I Have a Dream’ with the voices of the slaves. King began the speech itself by remembering the slaves and the fact that, despite the Emancipation Proclamation, they had never been emancipated. But even before that, King asked Mahalia Jackson to sing ‘Buked and Scorned.’ That old spiritual was a touchstone for King, a channel of his almost mystical kinship with the slave forebears. He was signaling that for all the civil religious trappings, he intended to read American history in the light of black history. You need to place that choice of song in the context of the early ’60s; long before black power, he’s bringing black cultural forms out into the public square.”