Unfortunately the unflattering comparison between us and them does not end with different levels of excitement. They outperform us in another, less likely area. On Monday, Barack Obama called for a truce in what is fast becoming a Democratic civil war on America's most radioactive topic: race. He and Hillary Clinton have been slugging it out over a series of remarks that make clear that - for all the optimistic talk after Obama's victory in Iowa - America's racial divisions remain live and open.

It began with Hillary's insistence in New Hampshire that deeds, not words matter. Martin Luther King made some fine speeches, she said, but there would have been no civil rights revolution had it not been for legislation driven by Lyndon Johnson. "It took a president to get it done," she said.

That offended plenty of African-Americans, who felt Hillary was downplaying the role of an American patron saint. Some detected a nastier subtext, implying that while black folks might do rousing oratory, it's the white boss who makes the rules.

To make matters worse, Bill Clinton launched a finger-wagging rant against Obama, culminating in a declaration that this is "the biggest fairytale I've ever seen". This line was surely directed at Obama's position on the Iraq war, but many saw it as a sweeping condemnation of the Obama candidacy, as if the very notion of a black president is fanciful.

Clinton's assorted outriders have not helped matters. New York attorney-general Andrew Cuomo (Mario's son) said that the early primary contests were tough, because you had to face the voters directly: "You can't shuck and jive at a press conference." That "shuck and jive" dates back to the 1870s and, says the dictionary, was an "originally southern 'Negro' expression for clowning, lying, pretence".

There's been further controversy about a line from my column last week quoting a senior Clinton adviser who told me that Obama appealed to those voters, especially the young, who wanted an "imaginary hip black friend". (I've been bombarded by pro-Obama emailers demanding I reveal my source.) Add to that the various Clinton surrogates who have brought up Obama's past - and openly admitted - drug use, and the Obama camp's charge that the Clintons are pandering to white voters, and you've got yourself an old-fashioned American race row. Last night, the candidates were due to meet in Las Vegas for a debate billed as a fight on "black and brown" issues.

None of this has been edifying, and many American commentators, coming down from their post-Iowa high, have lamented that race can still play such a toxic role in their politics. Democrats are particularly worried that what should be a great moment for the party - choosing between a woman and an African-American - will turn instead into a feud so bitter that it tears the Democratic coalition apart, wounding the eventual winner perhaps mortally.

And yet, viewed from here, there is some consolation Americans can take. Put simply: at least they care about this stuff. We cannot quite say the same.

The closest we Britons will get to a presidential contest will come in London's mayoral election in May. Up for grabs will be the largest individual mandate in western Europe outside the presidencies of France and Portugal. Chasing it is a trio of standout personalities: the incumbent Ken Livingstone, Tory columnist Boris Johnson, and the former policeman Brian Paddick.

Now imagine, just for a second, that this battle was being fought in the US. Then imagine that this weekend had brought confirmation, from one of the candidate's own allies, that he had described black people as "piccaninnies". And not just once, in a published article, but in conversation. In Africa.

In fact you don't have to imagine it. This week Boris pal Rod Liddle confirmed once more that Johnson, bored during a past trip to the Kenyan-Ugandan border, said to their Swedish Unicef hosts and their black driver: "Come on, let's get out and see some piccaninnies."

Johnson has also written of the "watermelon smiles" of African men and mourned that Nelson Mandela had merely replaced the minority tyranny of white rule with the "majority tyranny of black rule". In 2002 he wrote in the Spectator of that trip to Africa: "The problem is not that we were once in charge, but that we are not in charge any more ... the British planted coffee and cotton and tobacco, and they were broadly right ... If left to their own devices, the natives would rely on nothing but the instant carbohydrate gratification of the plantain."

Johnson has a variety of defences for this long-written record. He usually falls back on that politicians' standby, that his words were taken out of context; but read the full articles and, if anything, they sound even worse. Sometimes he mentions that he is one-eighth Turkish, as if that fragment of DNA inoculates him against all prejudice. Or his allies harrumph that this is political correctness gone mad and that it's Boris's daring to offend that has made him such a compelling read. I don't argue with that last assertion: the trouble is, Boris Johnson is not competing for a trophy in the British Press Awards, but to lead the most diverse city on the planet. The rules of this game are different.

Mainly, though, Boris doesn't like answering these charges at all. I have spoken to him once since he declared his candidacy, when he strode over to me at a party late last year, insisted that I stop spreading "poison" about his record, and told me I was "full of shit".

Again, it's worthwhile imagining how this would play in America. What if Michael Bloomberg had spoken that way as he campaigned to be the mayor of New York? What if a white candidate for high office had spoken of "piccaninnies" and had lamented that whites were no longer ruling Africa? Do we think he would have survived for five minutes?

In America we now know that even to compare a black candidate to Martin Luther King, done in the wrong way, can bring trouble. Yet here a white politician can have casually dabbled in the ugliest stereotypes and not even face censure. The black press and luminaries, including Lord Ouseley and Doreen Lawrence, have spoken out, but the bulk of the media ignore them completely. (Channel 4 is soon to give an hour to a Dispatches critique of Livingstone, with no equal scrutiny of Johnson.) The media prefers to indulge good old loveable Boris.

No one, I know, can seriously envy America's discourse on race. The question has crippled the country since its founding. Its history is tortured. And yet at least America takes note of the feelings of its non-white citizens. Here in London, a place more diverse than Iowa, New Hampshire, or even the United States itself, most of us hardly seem to care.

freedland@theguardian.com