To most of my European friends, an inexplicable aspect of American culture is the quixotic persistence and social influence of religious fundamentalism. They cannot understand how Americans could seriously consider for the second highest office in the land a candidate who has worshipped all her adult life at churches where congregants believe the literal truth of every word in the Bible and practise “speaking in tongues”. Thanks to YouTube, we even know that Sarah Palin has been blessed to protect her against witchcraft.

According to opinion polls, about one third of Americans subscribe to a literal interpretation of the Bible - from the chatty serpent in the Garden of Eden to the bloody prophecies in Revelation. They constitute a large and disciplined minority - a primarily Protestant army of Christian soldiers, with a pre-Enlightenment mindset and disdain for secular values.

Ironically, the US Constitution, which deliberately omits any mention of God, may be the primary reason why extreme forms of pre-Enlightenment Christianity have proved more tenacious in the US than in Europe. “Thou shalt not speak ill of anyone else's faith” is the eleventh commandment in a nation with ever-proliferating religious denominations, and views that would seem bizarre elsewhere in the world elicit polite silence in Middle America.

The term “fundamentalism” was not coined until the early 20th century. The 1925 Scopes “monkey trial” convinced many educated Americans that fundamentalism was on its way out. John Scopes, a biology teacher in Dayton, Tennessee, was convicted of violating a state law forbidding any mention of evolution in public schools. But Clarence Darrow, the nation's best known defence lawyer and a devout agnostic, made a fool of the prosecutor, William Jennings Bryan, a three-time Democratic presidential candidate. Bryan admitted, since he knew the Earth revolved around the sun, that Joshua could not have made the sun stand still at Jericho.

In Only Yesterday (1931), the historian Frederick Lewis Allen concluded that “in the hinterlands the pious might still keep their religion locked in a science-proof compartment of their minds; but civilised opinion everywhere had regarded the Dayton trial with amazement and amusement, and the slow drift away from Fundamentalist certainty continued”.

This proved a misjudgment of historic - one is tempted to say biblical - proportions. Allen and Darrow would have been astonished had a time-traveller told them that evolution would still arouse controversy in the US in 2008.

The word “fundamentalism” is rarely used in surveys of American religion, because it is considered a pejorative by many fundamentalists themselves. Pollsters usually ask if Americans consider themselves “evangelicals”, a less loaded term encompassing theological liberals and conservatives. Fundamentalists and evangelicals share a faith resting on a personal relationship with God, but not all evangelicals regard the Bible as literally true. Both Jimmy Carter and George W. Bush are evangelicals, but Bush's statements indicate that he is a fundamentalist while Carter, who has strongly supported the teaching of evolution, falls on the liberal side of the divide.

Another irony is that people who belong to no Church make up the fastest-growing segment of the American population. In the 1980s, no more than 8 per cent refused to identify a religious affiliation. This year, the Pew Forum on Religion & Public Life found that the ranks of the unchurched had doubled. Highly educated Americans are most likely to fall within a group ranging from atheists to those describing their religion as “nothing in particular”.

There is a powerful correlation between fundamentalism and lack of education. According to Pew, 45 per cent of Americans with no education beyond high school adhere to biblical literalism, while only 29 per cent with some university education - and 19 per cent of university graduates - share that old-time faith. Republicans have tapped into the fundamentalist resentment of educated, sceptical elites to form the party's right-wing Christian base.

The main difference between fundamentalists at the time of the Scopes trial and today is not their numbers - which may have decreased - but the political clout they have acquired within the Republican Party. In the past, fundamentalists did not hew to one political line. Bryan, known as the “Great Commoner”, was an economic populist who would be astonished to see his descendants allied with the party of 19th-century robber barons.

Another important difference is their strange - in view of the history of Protestant anti-Catholicism before John F. Kennedy's election in 1960 - alliance with the most conservative elements within the Roman Catholic Church. On issues such as abortion and gay marriage, the Catholic hierarchy (as distinct from the Catholic laity, which is much more liberal) and right-wing Protestants are the best of friends. John McCain, who has always aroused suspicion among social and religious conservatives, chose Mrs Palin as his running mate precisely because she is the kind of fundamentalist who melds religion with class resentment.

Fundamentalists represent a black-and-white value system. Because their beliefs matter so much more to them than religious indifference does to the religiously indifferent, they exert influence far out of proportion to their numbers.

Whether that influence will continue depends partly on the election results and partly on whether the alliance between right-wing Catholics and fundamentalist Protestants endures.

There will certainly be a seat at the table for religious Americans in an Obama Administration - but not for the fundamentalists embodied by Mrs Palin or the Catholic Supreme Court Justice Antonin Scalia, whose judicial opinions cite pre-Enlightenment views on divine authority as the source of governmental power. If Mr McCain is elected, he will be even more beholden to the Christian Right than George W. Bush. The next president will probably be able to appoint at least two Supreme Court justices, and lifetime judicial appointments influence American society for decades beyond the tenure of any president.

Susan Jacoby's latest book is The Age of American Unreason (Old Street Publishing, 2008)

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