I have just returned from the United States, where political insult and invective hit lows that would be considered beyond the pale in Britain. Race and sex stir deep emotions and there are undoubtedly deep hostilities in the presidential contest.

Barack Obama is the first viable black American candidate for the presidency. He has the wit to realise that if he panders to "special interests" and is seen as the candidate of the blacks, he has no chance of succeeding; thus his efforts to reach a wide audience have seen him characterised as "not black enough".

In Britain, we fail to understand how deep the fissures around race in America can be. Before the end of this election, all efforts will be used to discredit Obama. To some, the idea of a black president is still unthinkable.

Many of us are waiting to see whether Obama will add flesh in terms of policies to the brilliance of his oratory. But what cannot be denied is his huge intelligence. Last week in this magazine, Andrew Stephen suggested that "far from being the brilliant student . . . Obama was a consistently B-grade pupil", who ended up at a none-too-great liberal arts college before moving to Columbia University and then Harvard Law School. But this trajectory could not be achieved by a B-grade brain. Columbia is very competitive and places at Harvard Law School are highly prized.

Obama went on to become the first black president of the Harvard Law Review, the most prestigious of legal journals, which had been an exclusionary zone to women and blacks. He was the first black person to break the barrier.

Obama's political team has been criticised for allowing the media to interview his very elderly Kenyan "grandmother". Stephen wrote: "The only problem was that the woman in rural Kenya was not Obama's grandmother but the alleged foster mother of Obama's father." Obama has written about his father's foster mother, who was not his birth mother but was in every other respect his parent. It should not be presented as a manufactured relationship.

There have been suggestions that Obama's opposition to the war may be a recent invention since he was not able to vote in the Senate in 2002. But Obama was in the Illinois state legislature and, unlike Hillary Clinton, was highly vocal in his opposition to the war.

Nor is it true that there is little difference politically between the leading Democrats. There is an important difference. The only Democratic candidate who does not totally oppose "enhanced interrogation techniques" is Clinton. She has said there may be circumstances in which special methods of interrogation might be used on the authorisation of the president. Such a position is an assault on the absolute prohibition on torture. Politicians who betray their ideals to secure power rarely recover those ideals once in office.

Obama is now being patronised as a "kid" and a purveyor of "fairy tales" by Bill Clinton. These insults echo a past in which black people in America were not dignified with adulthood but were referred to as "boys".