George W Bush's daughter was married this month by the same minister who delivered the inaugural prayer for Bush in 2000 and 2004. He is Kirbyjon Caldwell, a Black, Houston-based leader of a megachurch who has publicly announced his support for Barack Obama. Caldwell has said that he will campaign with Obama. It's quite likely that Caldwell will replace Jeremiah Wright, who was Barack Obama's pastor until their very public split.

Why is Obama's public piety so important to his presidential campaign? With the middle name Hussein, a father reared as a Muslim, and his youthful attendance at a Muslim school in Indonesia, Obama has to prove his Christian credentials to Americans, 78.4% of whom say they are Christian (see the Pew Forum on Religion and Public Life for a comprehensive look at the place of religion in American politics).

But even if it weren't for his association with Islam in these Islamophobic times, he would still need to flaunt his religious credentials. Americans love public professions of piety from those running for election or after public disgrace. For example, the Republican presidential nominee, John McCain has been appearing with an influential radio and television minister, Reverend John Hagee, who publicly fulminates against gays, Catholics and who knows who else.

I often say that I find Europeans to be just as religious as Americans and much more likely to assume one is Christian. Parisians have asked me about my plans for Easter in Paris far more often than I've been asked in New York, and while I was never invited to a christening during a decade and a half in New York, I've already logged two during the five months I've lived in London. The difference is attitude: in western European countries like England and France, people are likely to pretend that they are less religious than they are. Americans claim that they are more religious than they are. Take the case of Tony Blair. According to one of the numerous memoirs published by his intimates, the former prime minister cloaked the frequency of his church going from the British public. He refused to admit whether he had prayed with George W Bush, which was a yes by omission.

In the United States, American presidential candidates make noisy protestations of faith. To some people in Europe, the need for American presidential candidates to make protestations of faith must seem quite retro. The U.S. is more akin to places like Morocco or England, where the clergy are an integral and powerful force in politics and each candidate must display public piety, or Poland, where the Catholic Church is likened to a third political party.

Why is religion - and consequently religious leaders - so powerful? Churches, mosques and synagogues are one of the few places where adults choose to gather weekly. Only school and work claim more regular attendance. These adults then willingly submit to a behavioural lecture by someone who claims to possess better, higher knowledge. And in the United States, ministers routinely tell their congregations who to vote for, making one's vote a question of morality and thus admittance to heaven. Despite the fact that church attendance in the United States isn't terribly high, estimated at only about 20% by those who have monitored congregations (20% is about half of what the figures are when people self report), churchgoers matter because they are primed for obedience and if they are told to go to the polls, they will.

In the black community, the church has always been the font of power and action out of necessity. During the years of legal segregation, it was one of the few places where blacks were allowed to gather freely. The young, just-out-of-law school Barack Obama was both morally and politically ambitious. He was drawn to work in the public interest because of his mother's focus on service. While his upbringing was rich in comparative religious education he had no religious affiliation. His mother thought that religion was not something one inherited but something one chose if so inclined.

Obama wrote that he was drawn to churches in Black communities because of "the power of the African American religious tradition to spur social change. Out of necessity, the black church had to minister to the whole person. Out of necessity, the black church rarely had the luxury of separating individual salvation from collective salvation. It had to serve as the centre of the community's political, economic, and social as well as spiritual life."

The safest route for any black man looking to be a powerful community force or a successful politician has been the clergy, a path taken by Al Sharpton, Adam Clayton Powell Jr., Jesse Jackson, Martin Luther King Jr. and numerous other figures of the civil rights movement. Fifteen years ago, in a highly-contested gubernatorial election in New Jersey, a Republican strategist bragged that he had paid Black clergymen to encourage their congregations to stay home - not to vote in the election (it would have been too implausible for the black clergy to encourage them to vote Republican). Church leaders quickly claimed it wasn't true. But way down South, a longtime Democratic supporter told me, "We pay the ministers to tell them to vote for us too."

Bush's expressions of faith seem heartfelt. He begins each day by reading the Christian Bible and praying. During his administration, Bible study is an important part of White House life, according to one of Bush's former speechwriters who felt marginalised because he didn't attend. But it might be that Bush's true piety enables him to see religious affiliation as more than a political end. Caldwell announced his support for Obama several months before Jenna Bush's wedding yet he still performed her marriage ceremony. He said that he had called Bush to tell him of the decision and that it wouldn't affect their relationship. Bush is a lame duck president with little to lose, but I began to think that he might be that rare elected figure whose relationship with religion does not fall prey to political expediency, and I shuddered with grudging respect.

The left in the United States has never figured out a way to publicly divide the issues of morality and religion. They must make it clear that a strong moral core can have no basis in religion. In many parts of Europe, environmental issues have become a counter morality to the church, but I predict that it will be a long time before a green party gains traction in the United States (Al Gore's interest in environmental issues has failed to endear him to the American public. Around the time he won his Nobel Prize, the American press pointed out that at his home in Tennessee, the Gores use more than twenty times the national average of energy for their family size).

Until the left manages to reclaim this moral space, each presidential candidate will go through the song and dance of public piety. Perhaps, it's time to think about venues that can claim the time that churches do. Is the internet one of those spaces?