Saturday marks the 49th anniversary of what historians widely regard as one of the greatest presidential speeches in American history.

On March 15, 1965, as the nation reeled from the "Bloody Sunday" beatings of civil rights marchers in Selma, Ala., President Lyndon B. Johnson made a stirring call upon Congress to ensure the voting rights of black Americans.

The address itself has quite the backstory too.

Richard Goodwin, the writer of what came to be known as the “We Shall Overcome” speech, composed it in a one day-dash to a deadline.

The Speech, In 8 Hours

"I speak tonight for the dignity of man and the destiny of democracy," is how Johnson began the speech.

He spoke to a nation sickened by the sight of state police and a sheriff’s posse assaulting civil rights marchers in Selma the week before. He had an audience of 70 million television viewers, who had seen for themselves clubs, bullwhips and tear gas unleashed against people trying to register black Americans to vote. The veil was off the injustice. The president was throwing his heft and force into the cause before a special joint session of Congress.

"It is wrong, deadly wrong, to deny any of your fellow Americans the right to vote in this country," Johnson said later in the address.

The voice belonged to Johnson. As did the conviction. But the lines were crafted by a young speechwriter who that day arguably pulled off one of the greatest deadline assignments in presidential history.

It was a Monday, and Goodwin wasn’t at his best. He had arrived at the White House on banker's hours after a late night of what he called “postprandial” drinks and socializing.

"And I got home and picked up my White House telephone, there were no messages for me," Goodwin said in an interview this month. "And I said, 'Good, I don't have to write it.' "

Former speechwriter Richard Goodwin, in his Concord home, on March 4 (Erika Lantz/WBUR)

Johnson had decided on Sunday to address Congress the next night. But when the pajama-clad president woke early Monday to the news his righthand man Jack Valenti had assigned the speech to someone else, he exploded.

Goodwin recalled: "Johnson looked at him and said, ‘You did what?!' He said, ‘Don’t you know that a liberal Jew has his hands on the pulsebeat of America? And you assigned this speech to a Texas public relations man?' He said, 'Get Dick on it!' "

Soon Dick was on it, with no time to spare. Now 82, the luminary remembers Valenti waiting for him at the office that day, jumping up and down. Goodwin had just eight hours.

He started channeling his own ideals and his experience of rabid anti-Semitic prejudice.

He wrote: "To deny a man his hopes because of his color or race or his religion or the place of his birth is not only to do injustice, it is to deny America and to dishonor the dead who gave their lives for American freedom."

With incandescent language and the goal of moving men to action at long last, Goodwin linked the marchers in Selma to the Minutemen of 1775 at the rude bridge in Concord down the road from where he now lives. He connected them to veterans who fought for the cause of freedom in Korea and World War II.

Author and historian Doris Kearns Goodwin says she fell in love with the speech — before she ever met the speechwriter, whom she later married.

"What you're calling forth are the memories of the people who came to America for a purpose," she said. "So it's part of what is a nation. It’s what Lincoln called on in Gettysburg."

Dick Goodwin had entered the White House at 29, as an aide to John F. Kennedy, before becoming Johnson's chief speechwriter. On that Monday, Goodwin had a rare opportunity at the heights of power to pour his passion into a presidential speech.

"Here there was no other side," he said. "Blacks had been marching for the right to vote. They’d been beaten up in the streets."

Goodwin's draft of LBJ's "We Shall Overcome" speech (Erika Lantz/WBUR)

He added: "Nobody was out there saying Negroes did not have the right to vote or the government should not help them. So all the morality was on one side. And I felt that. I believed that and I felt it."

A hundred years after Lincoln, he wrote, "Emancipation was a proclamation and not a fact," and the "Negro" struggle showed a faith in democracy that shamed the nation.

"A century has passed since the day of promise. And the promise is unkept. The time of justice has now come," the speech said.

While Goodwin was giving birth to those lines, everyone bore the brunt of the waiting president's impatience — except Goodwin himself. No one dared interrupt or delay him. Valenti would later call him "as loveable as a forlorn porcupine."

Valenti said once that Goodwin held on to a speech until the last moment, to prevent the speaker from messing around with it.

"And not just the speaker, but the rest of the White House staff," Goodwin added. "Everybody wanted to get their hands on a presidential speech, and if you left it to them enough time before it had to be delivered, who knew what the hell they could do?"

Pecking the keys of his manual typewriter with two fingers, Goodwin finished his last page too late for revisions or rewriting. He called those hours his finest in politics. After a ride in the president’s limo in a rumpled suit, Goodwin got to stand in the well of the House and see the president raise his arms for emphasis as he delivered what became the most famous line:

Their cause must be our cause too. Because it's not just Negroes, but really it's all of us who must overcome the crippling legacy of bigotry and injustice. And we shall overcome.

'A Transformative Moment'

State troopers swing billy clubs to break up a peaceful civil rights voting march in Selma, Ala., on March 7, 1965. Now-U.S. Rep. John Lewis is the protester on the ground in the foreground. (AP)

In Selma, Ala., John Lewis watched in amazement. His skull had been fractured by a police club a week before. Now he sat beside Dr. Martin Luther King Jr.

"And tears came down his face. Dr. King started crying and we all cried," Lewis told NPR a few years ago. "And Dr. King said to me, 'John, we will make it to Montgomery and the Voting Rights Act will be passed.' "

"There was a sense of profound emotionalism about this," said historian and LBJ biographer Robert Dallek. "And for Johnson to get up ... and adopt the anthem of the civil rights movement, especially for those who had been battling ... for decades, it made them cry. People understood that this was a transformative moment."

Senators and congressmen interrupted the speech 40 times with applause, at times standing and thunderous. If the goal of political speech is to move men to action, this was Johnson's — and Goodwin's — finest hour. Slumped in their chairs or sitting on their hands, the Southern Segregationists knew they were done.

Five months later, Johnson signed the Voting Rights Act of 1965 and gave Goodwin the pen that's now on his wall.

"Yes, it's Richard Goodwin's words, but it's very much Lyndon Johnson's speech," Dallek said.

It is the custom of presidential speechwriters to disavow ownership, yet Goodwin would join Johnson that night in the presidential quarters.

"Johnson had no desire to go to sleep," Goodwin said. "He knew he had had a great triumph."

The two of them stayed up drinking scotch until 3, hoping the applause would never end.

- Watch the full speech here: