It took Barack Obama a full 15 days to comment on the death of Trayvon Martin, the unarmed, 17-year-old African-American boy wearing a hoodie who was shot dead by a trigger-happy vigilante in a Florida gated community. The president resisted increasingly loud calls to condemn what appeared to be a plain case of racially motivated murder. Then, when he finally spoke, he made the killing personal, adopting the dead boy as if he were his own with the words, "If I had a son, he'd look like Trayvon."

Why the hesitation? The answer, of course, is that the US remains cursed by race. Even in a cosmopolitan metropolis such as New York, the claim that America is a racial "melting pot" is wishful thinking, a contrived but powerful myth that belies the cruel reality that 150 years after the civil war and more than 50 years since Lyndon B Johnson's civil rights legislation, Americans remain divided by the colour of their skin.

There are some encouraging signs that the worst days of African Americans being treated as inferior are gone. It is significant, for instance, that the death of Trayvon Martin has become a national tragedy and a source of universal shame. Yet it was still almost two weeks between the day he was killed, 26 February, and the story breaking nationally on 8 March. Then a further fortnight passed before the president decided to speak.

Code-breaking

It was hoped that Obama's presidency would prove a milestone on the road to a post-racial US, yet there are clear signs that race continues to poison the political debate, even at the highest levels. Liberals list the "dog-whistle" phrases used by prominent Republicans to encourage their racist supporters. Like all attempts to prove the use of subliminal codes, it would take the genius of Alan Turing to crack the secret cypher for certain.

When Rick Santorum told Southern audiences that "America was great before 1965", was he really signalling to the heirs of the Confederacy that the US was a far better place before the civil rights march that year, led by Martin Luther King, Jr through Selma, Alabama, that ushered in LBJ's Voting Rights Act? When Santorum was caught on camera saying, "I don't want to make black people's lives better by giving them somebody else's money; I want to give them the opportunity to go out and earn the money," was he caught being racist? Santorum hastily denied he had said "black people", insisting he'd said "blah people" instead. Blah? Blah-blah-blah.

When Arizona's governor, Jan Brewer, was seen testily wagging her finger at Obama before announcing he was "thin-skinned" and that she "felt a little threatened, if you will, in the attitude that he had", was she, too, reinforcing primitive attitudes she dare not openly admit? When members of the birther movement obsess about whether Obama was American-born and hint that he is a closet Muslim, are they motivated by their dislike of the president's mixed race? When Obama called out Newt Gingrich for labelling him "the food stamp president", was Gingrich, as the president suggests, "tapping in to some of our worst instincts"?

Republicans say Democrats accuse them of being racist to shut down debate about issues such as welfare dependency and voter fraud. When Juan Williams, an African-American Fox News commentator, tackled Gingrich at a candidates' debate on whether using language such as "entitlement society", "poor work ethic" and "food stamp president" was an incitement to racism, the Republican crowd roared its disapproval that the question had even been asked.

Lee Atwater, who advised both Reagan and George H W Bush, was in no doubt that racism informs Republican campaigning. "You start out in 1954 by saying, 'Nigger, nigger, nigger.' By 1968 you can't say 'nigger'. That hurts you. Backfires. So you say stuff like forced bussing, states' rights and all that stuff," he confessed.

David Gergen, who advised Nixon, Ford, Reagan and Clinton, is also adamant that Republicans use dog-whistle techniques. "There has been a very intentional effort to paint [Obama] as somebody outside the mainstream, 'Other', 'He's not one of us,'" he explained. "There are certain kinds of signals . . . code for, 'He's uppity. He ought to stay in his place.' Everybody gets that who is from a Southern background." And Gergen was describing the tactics of John McCain, who in turn suffered the racist smears of George W Bush surrogates who whispered he had fathered his adopted Bengali daughter out of wedlock.

Little wonder that Obama was slow to express sympathy with Trayvon Martin's parents. When he intervened on behalf of his Harvard pal Henry Louis Gates, the African-American professor arrested for breaking into his own home when he lost his front-door keys, the president accused the police of acting "stupidly" and said that "there's a long history in this country of African Americans and Latinos being stopped by law enforcement disproportionately". Such hasty remarks cost him dearly and he has been wary of taking sides in racial disputes ever since.

Bradley effect

Obama already suffers electorally from being an African American. An AP-Ipsos poll in the 2008 election showed that 6 per cent more Americans would have voted for him had he been white, and that is without taking into account the "Bradley effect", the phenomenon which suggests that, even in anonymous polls, racists are so reluctant to be thought racist that they lie about whether they would back a black candidate.

With the economy still failing fully to ignite, and with his likely opponent, the oddly orange-hued replicant Mitt Romney, prepared to say or do anything to get elected, Obama will need every last vote to win in November. Doing the right thing, as Spike Lee would put it, and letting Americans know he feels Trayvon's parents' pain may not prove to have been the wisest move. Better perhaps to disappoint supporters you know will vote for you come what may than risk being seen to side with one race over another.

Nicholas Wapshott's "Keynes Hayek: the Clash that Defined Modern Economics" is published by W W Norton (£18.99)