The American writer James Bryce once pondered why so few of his country's presidents were great or striking figures, men of brilliant gifts. (He was writing towards the end of the nineteenth century, so it presumably did not occur to him that a woman could ever be president). If we fast forward around 120 years, Bryce's musings remain all too apposite. Too many US presidents have been flawed men who failed in office. Enoch Powell once remarked that all political careers end in failure, but it certainly does not need to be so.

Politics in America, and to some extent the world (for the US presidency is the world's most powerful office) will be dominated in the coming year by the race to succeed George W Bush, a president who has failed more grievously than most.

Hundreds of journalists, pollsters and assorted political hangers-on and hucksters are already prowling round the cold and slushy byways of the usually obscure midwestern state of Iowa, because early in the New Year the state's caucuses will give crucial early clues as to who the Democrat and Republican candidates will be.

Failure in Iowa does not necessarily kill a campaign, but success can certainly kick-start one. Mike Huckabee, the folksy Baptist, has appeared from nowhere - well, from Hope, Arkansas, to be accurate - as the unlikeliest of Republican front-runners, and he can show the world he is not a passing shadow. On the Democrat side, Barack Obama, for me by far the most interesting of the candidates, can defeat his main rival Hillary Clinton and give his already impressive campaign some really serious momentum.

The Republican Party has for many years had a populist, religiously conservative strain. As a Christian this never pleased me, for I reckon the intrusion of evangelical certainties into the political debate tends to do Christianity a disservice.

Indeed, the Republican Party had seemed to be shedding religion as a key element in its politics: candidates such as Rudy Giuliani, Fred Thompson and John McCain have not made much of their religious views and practice. But Huckabee's main rival in Iowa, Mitt Romney, is a devout Mormon. This makes for a potentially explosive contest, as southern evangelicals such as Huckabee can be very hostile to Mormonism, which they regard as a cult. The tensions will not be confined to religion. Romney is an old-style Republican patrician. His father was governor of Michigan; he is Harvard-educated and an extremely wealthy man. He could be regarded as a grandee, an elitist.

By contrast, Huckabee is from the rural, blue-collar badlands; a man who had to scrap for everything when he was young. In that sense he represents the American dream better than the smooth, bland Romney. This brings an element of chippiness - almost class politics - into the contest.

As for Clinton and Obama, they are fighting a close and increasingly fractious battle for endorsement in Iowa. Earlier this year I read Obama's remarkable memoir, Dreams From My Father. I was not too impressed by his description of his trip to Kenya in a quest to discover his African roots - that verged on the sentimental. What I admired was his modest, realistic and very moving account of his work as a community organiser in some of the worst areas of Chicago. Here was someone who truly cared about helping people, but at the same time understood how difficult and slow progress often has to be.

Obama is capable of electrifying oratory and he can write prose of beguiling beauty. Of course, this is not enough, but he also has freshness and decency. Even his inexperience is an asset, when you compare him with the more hard-bitten Clinton. Now the naysayers are saying he knows nothing of foreign affairs, he has not met many world leaders, and so on. So what? George W Bush met many world leaders before he became president, and this did not prevent him from leading his country to disaster.

In the course of this century, the US presidency will diminish in significance, as new global superpowers such as India, China, a revitalised Russia and perhaps even Brazil become evermore significant. But for the present, America - not least because of its myriad battalions, its colossal ordnance and its vast armadas - is the country that matters most. The catastrophic current US president has made mistake after mistake, and he has damaged far more than his own country.

We have just over a year before the new president assumes power and grasps the hopes not just of America, not just of the West, but of decent human beings across our entire planet. Barack Obama is the only one who seems a deserving repository of all this amassed human hope.