When George W Bush's plane touched down in Pennsylvania early in the election campaign, a Catholic priest was there to greet him and share a silent prayer on the tarmac. For all Bush's religiosity, the reason was a very earthly one. Pennsylvania has 12 million Catholics, more than any other US state. And in each of the past eight presidential elections, the winner of the Catholic vote has also won the popular vote. Moreover, John Kerry is the first practising Catholic to run for president since 1960, when Jack Kennedy won 80 per cent of the Catholic vote.
Kerry can only dream of repeating Kennedy's success, but the Catholic vote has long been an important barometer of the Democratic Party's health. In the early 20th century, Catholic immigrants helped to broaden its basis of support beyond its southern bunker. In 1928 the surge of Catholic support for the Democrats' Catholic candidate, Al Smith, presaged the election of Franklin D Roosevelt four years later.
The northern, urban Catholic vote - coupled with the Democrats' traditional strength in the white south - formed the bedrock of the New Deal coalition which dominated US politics for all but eight of the following 36 years. Throughout this period, Democratic presidential candidates won much higher support among white Catholics than among white Protestants. (Blacks, overwhelmingly Protestant, had by the mid-1960s become the Democrats' most loyal voters.)
Yet during the 1970s and 1980s the Republicans secured a majority of the Catholic vote in a string of presidential elections. Crucial to this success was the Republicans' ability to capitalise on the rise of "values" issues - such as crime, abortion, patriotism and school prayer - which challenged the usual electoral dominance of economic issues.
Bill Clinton refocused the minds of "Reagan Democrats" on the economy. In 1992, his support among white Catholics was 11 points higher than among white Protestants; in 1996, it was 17 points higher. In 2000, says Deal Hudson, a Bush adviser and editor of a conservative Catholic magazine, "We realised that without the Catholic vote there was no way Bush could get elected." Despite early stumbles - such as his appearance during the primary season at the anti-Catholic Bob Jones University - Bush managed to fight Al Gore to a virtual draw for the Catholic vote, with non-Hispanic white Catholics supporting him by 52 to 45 per cent. His opposition to abortion helped, but so did his "compassionate conservative" message of help for the poor through increased federal spending on faith-based initiatives.
So what will happen this time? Polling suggests that Kerry has opened a narrow lead over Bush among Catholic voters. This is explained, in part, by the Hispanic vote. Hispanics are the fastest-growing sector of the Catholic Church in America and, if they vote at all, they opt overwhelmingly for Democrats. But also important, as Michele Dillon, of the University of New Hampshire, argues, is that "whether conservative or liberal, the vast majority of American Catholics are selective with what it means to be Catholic". In that sense, Kerry - with his pro-choice stance and his support for gay civil unions - is on the majority side of Catholic opinion. A survey shortly before the 2000 election found that majorities of Catholic voters supported both abortion rights and allowing doctors to assist the suicide of the terminally ill. Other polling indicates that most Catholics believe you can divorce, have an abortion, use birth control or be gay and still be considered "a good Catholic".
Bush has a lead of roughly 30 points among "traditionalist" Catholics but they amount to only a quarter of the total. As one pollster has shown, the overwhelming majority of "centrist" or "modernist" Catholics are more exercised by issues such as Iraq, the economy or healthcare when they vote than by the conservative social issues that preoccupy the Church. That may yet prove to be Bush's undoing.
Robert Philpot is editor of Progress magazine