Jesse Jackson has no difficulty remembering his last conversation with Martin Luther King. He was in the car park of the Lorraine motel in Memphis and King was standing a floor above him on the balcony outside room 306. The leader of the civil rights movement and his young apprentice were preparing to go to dinner with a local pastor, and King admonished Jackson for not wearing a tie.
"I said to him: 'You know, Dr King, the requisite for dinner is an appetite, not a tie.' And he laughed at that." Then King turned to Ben Branch, a musician standing next to Jackson, and asked him to perform his favourite song at a rally later that night: "Make sure you play Take My Hand, Precious Lord. Play it real pretty." Those were his final words.
The shot rang out a second later. "Next thing I heard," says Jackson, "someone was saying 'Get low! Get low!' The police were coming towards us with guns drawn. I remember crawling to the steps. There was blood everywhere, and a photographer scooped up a cup of it. It was eerie."
King was slumped against the rail of the balcony. His close friend Ralph Abernathy was beside him crying "Martin, Martin, it's going to be all right," but Jackson remembers thinking he was clearly dead on impact. Jackson took what felt like a "long, long, walk" to his room just next door and called King's wife, Coretta. "I told her Dr King had been shot, I think in the shoulder, and she should come over straight away. I didn't have the strength to tell her what I thought I had seen."
Those few moments, 40 years ago tomorrow, are carved not only into Jackson's memory but into the collective narrative of America. They certainly changed his life, driving an already keen activist, then 26, deeply and irreversibly into the pursuit of racial justice. "It put on us a burden not to stop, to say we must not allow one bullet to kill a whole movement," he says now. But in a wider sense they changed everyone else's reality too. They were what he calls "a defining moment in American history".
Jackson was on stage with King in the Mason Temple in Memphis the night before he was assassinated, when King made his famous "mountaintop" speech. King talked with almost uncanny premonition about the "threats out there" and what might happen to him at the hands of "some of our sick white brothers".
"It doesn't matter with me now," King concluded in words that will forever reverberate, "because I've been to the mountaintop ... And I've looked over. And I've seen the promised land."
Forty years on, Jesse Jackson is still weighing up how far up the mountain African Americans have ascended. That question is rapidly becoming a central debate within the Democratic party's search for a presidential nomination. Barack Obama, responding to sniping from his rival Hillary Clinton over his relationship with the outspoken pastor the Reverend Jeremiah Wright, put race centre stage in the election in a speech in which he positioned his campaign in a continuum that stretched back through the civil rights era of King to the fight against slavery. "We set forth at the beginning of this campaign to continue the long march of those who came before us, a march for a more just, more equal, more free, more caring and more prosperous America."
As someone who has stood at the front of that march for four decades, Jackson believes that there remains some way to go. Politically, he says, African Americans have come a long way. In 1965 there were about 300 black elected politicians in America; now there are some 10,000.
He sees a substantial step forward too in other areas of public life, notably entertainment and athletics. "Take the Super Bowl between the [New York] Giants and the [New England] Patriots. Millions of people watched the game and race never came up. It was the red team versus the blue team. That has detoxified racial relations."
But there are several headline figures that tell a very different story. A quarter of African Americans live in poverty, compared with 8% of whites; seven out of 10 black boys drop out before finishing high school; in several states such as New Jersey black children are 60 times more likely to be expelled from school than whites; there are more black men in jail than in college; and the most shocking statistic: on average black men die more than six years earlier than their white equivalents.
"People of colour have higher infant mortality rates, shorter life expectancy, greater unemployment, less education. We pay more for less. That is our characteristic 40 years later."
Jackson has an explanation for this piercing disparity between the advancement of black people in many areas of public life coupled with stagnation or even deterioration in their overall standards of living. He points out that the legal underpinning of Jim Crow segregation in the South was negated in a series of landmark legal actions starting in 1954.
Then the civil rights movement led by King began the process that would remove cultural impediments to black progress. But the issue with which he had only begun to grapple at the time of his death - economic inequalities experienced by black as well as white working-class communities - remains a festering sore.
"Yes, blacks have been elected to political office, but the economic problems that King had started to turn to are still there. The result is you get black mayors presiding over the worst inner-cities in America, where jobs are leaving, investment is leaving, guns and drugs are coming; where there are first-class jails and second-class schools."
Despite this double-edged message, Jackson sees the Democratic race for the presidential nomination this year as a cause of real rejoicing. "When I see men voting for a woman and whites voting for a black in great numbers, that is a rebirth for America."
He singles out the recent primary in Mississippi, a state made notorious by a succession of murders of civil rights activists. "This was where Emmett Till was killed, Medgar Evers was killed, James Chaney, Andrew Goodman, Michael Schwerner were all killed. And now Mississippi holds a national Democratic primary, not a white primary, and an African American wins. That was a great moment in American history."
Jackson himself made two presidential attempts, in 1984 and 1988, and though he won in five and 11 contests respectively he failed to gain the Democratic nomination. "Whites were reluctant then to break the culture barrier," he says.
He remembers a night speaking at a rally in a cornfield in Iowa, and after the event an old white man came up to him and said: "Reverend, you have a good message, but we are just not there yet. But don't give up on us."
"That has never left my mind," says Jackson.
Obama has paid homage to Jackson as the man who made his presidential run in 2008 possible. Jackson accepts the compliment, and articulates it: "I went around and kissed the hands and held the babies. And those babies are now growing up and voting. So we are seeing the maturing of America; the country is growing up politically."
If you put all these disparate elements together, the positive and the not so positive, how far has America come since that terrible night at the Lorraine motel? "There is unfinished business," replies Jackson. "But when you see an African American who came from a housing project in Chicago now governor of Massachusetts, when you see 42 blacks in Congress, and Barack Obama leading in delegates and the popular vote to be presidential nominee, that is some of the promised land beyond the mountaintop."