(CNN) -- The Rev. Bernard LaFayette Jr. was resting at his Chicago, Illinois, home one autumn weekend in 1967 when the phone rang. The caller didn't identify himself, but LaFayette immediately recognized the baritone voice.
"Bernard, I need you," the Rev. Martin Luther King Jr. said. "This may be my last campaign. We're going for broke."
Most Americans think of King as the "I Have a Dream" preacher at the Lincoln Memorial in Washington. But the man who made his final trip to Memphis, Tennessee, in 1968 had become radical, scholars and activists say. King was gambling his legacy on a final crusade that was so revolutionary, it alarmed many of his closest advisers. Some became concerned about his emotional stability.
King called his crusade the Poor People's Campaign. He planned to march on Washington with a multiracial army of poor people who would build shantytowns at the Lincoln Memorial -- and paralyze the nation's capital if they had to.
The campaign's goal: force the federal government to withdraw funding for the Vietnam War and commit instead to abolishing poverty.
What King was saying by this time was even more provocative than what he planned. In his final presidential address to the Southern Christian Leadership Conference, he said the movement should address "the question of restructuring the whole of American society."
He called for a guaranteed annual wage for all able-bodied people, he urged the nationalization of some industries, and he told people to "question the capitalistic economy."
"It didn't cost the nation one penny to integrate lunch counters ... but now we are dealing with issues that cannot be solved without the nation spending billions of dollars and undergoing a radical redistribution of economic power," King said during a trip to Mississippi in February 1968.
The campaign was so risky that King told LaFayette, a Southern Christian Leadership Conference leader, during their phone call that he was going to appoint a new layer of executives to the civil rights group he co-founded.
"He was anticipating that we might be hit with some assassinations, so he wanted somebody left to assume responsibilities to keep it going," said LaFayette, who was appointed director of the Poor People's Campaign.
Taylor Branch, the Pulitzer Prize-winning author of "Parting the Waters: America in the King Years," said King didn't expect the crowds in Washington to embrace his vision of economic equality. He expected violent reprisals from troops. He might die. Yet King hoped that the sacrifice would lead to an economic bill of rights for poor people.
"When he did the Poor People's Campaign, he knew it wasn't likely to win," Branch said. "It was a witness."
But it was a witness that few people were prepared to hear, said Roger Wilkins, a U.S. Justice Department official designated as the liaison between King's final campaign and the federal government.
"By 1968, a lot of white people had gotten tired of civil rights and thinking of race," Wilkins said. "The picture of docile black people holding hands and singing freedom songs had been replaced by images of poor blacks rampaging through cities, looting and burning."
King had also lost the ear of his most important ally, President Lyndon Johnson. On April 4, 1967, exactly a year before he was assassinated, King delivered a highly publicized speech against the Vietnam War.
"Johnson was outraged," Wilkins said. "He turned sour toward King and the movement. He felt that Martin had rejected him."
The Southern Christian Leadership Conference, King's own organization, withdrew support from him. The group's board of directors voted against publicly backing King's opposition to Vietnam. Other black civil rights leaders criticized King as well.
"There were some black preachers telling him he was out of his element," LaFayette said.
King became depressed at times, Branch said. One night, King -- alone with a whiskey -- awakened friends in adjoining hotel rooms with his shouting: "I don't want to do this anymore! I want to go back to my little church!"
"The shameful truth is that very few people were paying attention to him," Branch said.
King mused about getting out of the civil rights business. He considered the idea of becoming dean of the chapel at Boston University, his alma mater, Branch said.
"He was constantly saying, 'Oh, I wish I could do this,' but he could never do it," Branch said. He was just possessed by the movement."
Yet King's evolution opened alliances with new supporters such as anti-war activists, said the Rev. Vincent Harding, an author and friend of King's who helped write his 1967 speech denouncing the Vietnam War.
"Some people were backing off at the same moment that there were other kinds of people who now recognized that King was not there for black people but for a new American society," Harding said. "Those who wanted to work for this new society were seeing him as a hero."
What this new American society could have looked like under King's leadership is unclear. He never got the chance to lead his final crusade.
He was assassinated in Memphis on April 4, 1968, while helping lead sanitation workers on strike.
The Poor People's Campaign has faded from historical memory. It remains the most overlooked part of King's legacy, Wilkins said.
It remains in the shadows because King rewrote the traditional civil rights script, Wilkins said. As long as he fed Americans images of bigoted Southern sheriffs clubbing demonstrators, people could remain comfortable. But the Poor People's Campaign gave Americans a new cast of villains: themselves. Americans didn't want to look at the face of poverty, but King was going to force them, he said.
"When the movement was just about the South, you weren't rattling the status quo," Wilkins said. 'You were doing things that made Northerners feel morally superior to the South."
LaFayette last saw King on the day he was assassinated. At the time, King was still thinking big. He told LaFayette that he wanted to globalize nonviolent protests.
King may have been isolated and dejected during those last days, but that's not the man LaFayette remembers. He takes comfort from one of King's final moments: the "mountaintop" sermon King gave the night before he was assassinated.
"You could see it in his eyes; he was consumed with passion," LaFayette said. "He was prepared. They didn't take his life. He gave it up. They didn't have to run him down and try to catch him. He was standing tall despite the threats.
"You can't take a person's life who's already given it up."