It is no accident that Barack Obama travels with a Secret Service detail. Such a level of protection is unprecedented for a candidate who has yet to win his party's nomination, but a plausible black American challenger for the presidency is also without precedent. The burly men who talk into their sleeves are there for a reason.

It isn't discussed much, not publicly. If you scan the American media you get the sense, in fact, that too much talk would be counterproductive, that it might even encourage sweaty, unspeakable types to seek their 15 minutes of fame. Besides, Mr Obama has worked hard to present himself as a candidate capable of transcending old divisions. The attentive Secret Service men operate from different assumptions.

The unstated reason can be dated: April 4, 1968, 40 years and a day ago. It can be traced to a nondescript place, now a kind of shrine: the Lorraine Motel, Memphis, on the balcony outside room 306. It can be given a context in the career of a preacher of non-violence who said that America was better than racism, a Christian executed for his temerity, his eloquence and his blackness.

A single sniper's bullet killed Martin Luther King. James Earl Ray was held to have acted alone: many did not believe it. But even a child in the 1960s could get the sense that someone, or some thing, in America had determined to kill them all, the Kennedys, King, Malcolm X, selected Panthers, George Jackson. The sole thing they had in common was a moral objection to racism, but still they died. And even four decades on, as Mr Obama's contest with Hillary Clinton goes to the wire, no-one is prepared to pretend that a black leader is safe.

Dr King believed deeply in God, racial harmony and peaceful protest. Yet even his Southern Christian Leadership Conference could be categorised as a "black nationalist hate group" by the swinish J Edgar Hoover and the FBI. Even before King's faith became "politicised" and he began to speak out against the Vietnam war, he was portrayed, routinely, as a dangerous radical "causing trouble" by opposing segregation and poverty. Those who assigned the Secret Service to Mr Obama know their American history.

Some strands come together. One connection involves a brutal truth. An African-American would not be aiming for the White House today had it not been for the murder of Dr King. Jesse Jackson cleared the road, somewhat, in 1984 and 1988, but the killing in Memphis marked the beginning of a long, necessary journey for America. Mr Obama dislikes being defined by his race, but his race, as much as anything, has caught the world's imagination. The world asks: would the United States really choose a black President?

The fact that the question is asked, four decades on, makes another connection. There may be 44 black people in Congress; African-Americans may be prominent in all walks of life; overt racism may be shackled by law and the certainty of an outraged reaction. But black America did not need the aftermath of Hurricane Katrina to know where it stands.

It stands, one-quarter of it, in poverty. It stands, in grotesque disproportion, in the prison yards. It stands atop the high school failure statistics and in the records of premature mortality. Currently it stands chief among the victims of sub-prime mortgage crimes. Dr King and the civil rights movement succeeded in changing laws, but failed to alter the realities that elude legislation.

Such is the legacy that challenges Mr Obama. Implicit within the fact is another measure of subtle but real racism. This candidate cannot afford to be a black champion and nothing more. He must "reach out to" - which is to say, fail to alienate - white America. He must not threaten. When he speaks of race he must be "inclusive" and invoke his mixed parentage. He must also be secure, as they say, in his faith.

That last trait did not save Dr King, of course, and will never persuade those guarding Mr Obama to relax. By all accounts it is not helping the candidate much in Pennsylvania, either, where the white working class are resisting his charisma. Instead, the Clinton camp, with a devout candidate of their own, remind voters of the Rev Jeremiah White, a radical preacher with a melodramatic turn of phrase whom Mr Obama has been obliged to rebuke for saying God damn to America's racism. To the Clintonites, this means merely that their rival is "unelectable".

Perhaps he is. Perhaps too little has changed since Dr King was murdered. Perhaps nothing important has changed. On the face of it, that thought makes no sense: Barack Obama could not have got this far without millions of white supporters. They say he has galvanised young America, white and black together. The polls continue to claim, nevertheless, that he would lose to John McCain in November, meaning he would fail to win enough white voters.


Still Mr Obama is considered at plausible risk of assassination, despite every reassuring word, despite refusing to be contained or detained by race, and despite his religious faith. He follows in Dr King's footsteps - he is, if anything, even less threatening than the pacifist - but still the Secret Service worries. The thinking goes something like this: if even the godly Dr King could be murdered, any black man can be murdered. Even now.

Speaking in Westminster Cathedral on Thursday night, Tony Blair, newly Roman Catholic, was, meanwhile, paving the way for his Faith Foundation by arguing that religion could "advance humanity" and help to end global poverty. Treat it as something "far removed from the necessities and anxieties of ordinary life", he said, and you grant an easy target to "militant secularism".

Put aside that while in office Mr Blair obscured the target, and his own faith, by refusing to "do" religion. Forget to ask how his moral sense will aid him (or the UN's millennium goals) when he is next asked to pass judgment on a war. The former Prime Minister seems intent on finding a place for religion in public life. Yet what is one thing we know for sure about America? Simply this: that no-one who disavows faith has any hope of public office. (So much, perhaps, for militant secularism.) But will faith keep Mr Obama safe?

It might have if he were white. Mrs Clinton says she is devout. Mr McCain is OK with God. President Bush appears to feel he has a hotline, indeed, to the Almighty. Religious observance is ubiquitous, but it has not even begun to conquer racism. In "one nation under God" it is difficult, indeed, to overlook the idea that when faith is taken for granted among politicians it is also discounted. It appears - and I'm trying to be gentle about this - to make no useful difference.

Martin Luther King would have disagreed, of course. Mr Obama and Mr Bush might find the idea ridiculous. But if I believe a recent Economist poll, I know two things. First, 80% of Americans declare a belief in God; secondly, the African-American candidate for the presidency is under strict Secret Service protection 40 years after Dr King's bloody murder. Let's call that a contradiction.