March on Washington set the standard for songs of protest

"When you look at music and its role in American history," Santelli said, "you can go all the way back to pre-Revolutionary War days, and you find that radicals in Philadelphia, Boston and New York were selling broadsides, which put new lyrics to English beer-drinking songs. This was a way to get the word out, to get your political slant promoted."

But there are times, he said, "when it becomes more important, times when it becomes more prominent."

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That was certainly the case with the March on Washington, where music also played a role behind the scenes.

Today the march is largely remembered as the day King gave his celebrated "I Have a Dream" speech from the steps of the Lincoln Memorial. Historians have long known that that portion of the talk was not included in the speech as written, but that King added the "Dream" section, which he'd voiced in previous talks, at the spur of the moment.

What's less well-known is what prompted King to depart from his text at that moment. Belafonte, who was on the platform that day, recalls King taking a pause during which gospel singer Mahalia Jackson shouted, "Tell them about the dream, doctor!," at which point he extemporized one of the most celebrated speeches in American history.

As civil rights activist Roger Wilkins later put it: "If Mahalia Jackson, with that voice, told you to do something, you did it."

Twitter: @RandyLewis2