A group of American evangelicals are on a mission to put moral issues at the heart of the most intense presidential campaign in years. Those used to the thundering rhetoric of the religious right will expect a tirade against abortion and gay marriage. But in an interview this week Jim Wallis says: “Top of the list is what happens to the poor. Poverty is the principal Biblical political issue.” He adds: “climate change is now a mainstream evangelical issue. Human rights, Darfur too.”

Wallis is one of the leading figures in a group known as progressive evangelicals, or Red Letter Christians. They are theologically conservative, but strikingly radical on poverty, the environment and Iraq. Inspired by William Wilberforce and Martin Luther King, they believe theirs can be a popular movement to transform US society.

There are signs too that they are making themselves felt politically, and are beginning to close the notorious God gap - the huge Republican lead among regular church-goers. In this year's open and unpredictable election, that could be crucial.

Evangelicals are about a quarter of the US electorate. Distinguished by a belief in salvation through a personal relationship with Jesus Christ and the infallibility of the Bible, they have been the backbone of the religious right. Red Letter Christians are no different theologically; the name comes from those traditional bibles where Christ's words were written in red, and is meant to emphasise a focus on his teachings.

But they say it is precisely this focus on Christ's message which gives them their radical edge. Wallis has written that he found the Sermon on the Mount “more revolutionary than anything I had read in Karl Marx, Che Guevara or Ho Chi Minh”.

“There are 2,000 verses in Scripture that deal with poverty - this is the dominant social concern,” says Tony Campolo, a professor at Eastern University and author of Red Letter Christians. From these verses stems a passionate commitment to social justice. From Jesus's ideals of peace, comes a heartfelt opposition to the Iraq war. “War is an ethical issue,” says Campolo. “You can't foster a war mindset and call yourself a Christian.” And scriptural verses emphasising's man stewardship over nature are the spur to take action on global warming.

Wallis says he used to be told that progressive evangelicalism was a contradiction, but now “a misnomer has become a movement”. His latest book, The Great Awakening, went straight into the bestseller lists, and he believes a younger generation of evangelicals is hungry for his message; “at every evangelical college school we go to on this book tour they run out of chairs.”

Older evangelical leaders, he says, are also embracing a new agenda. Certainly, with the passing of the old guard of partisan evangelical leaders, such as Jerry Falwell, a new generation of pastors has widened its moral focus, even those who would not classify themselves as progressive.

Rick Warren, pastor of the Saddleback megachurch in California and author of the 25-million selling The Purpose Driven Life, rated abortion, gay marriage and medical ethics as the central moral issues of the 2004 election. But he has since dramatically refocused on an energetic campaign on poverty and Aids in Africa. Richard Cizik, an otherwise traditionally conservative vice-president of the National Association of Evangelicals, has led attempts to make the environment a religious issue by defining it as “creation care”.

Wallis believes that this is the start of a social movement that will force politicians to embrace new priorities, just as King and Rosa Parks forced President Johnson to act on civil rights. But for Democratic party strategists, there is a more immediate opportunity this autumn. Many believe that a failure to reach out to religious voters cost Democrats the 2004 presidential election, in which weekly church-goers voted Republican 58 to 41 per cent (and 64 to 35 for those who worship more often).

The candidates are not making that mistake this time. Although Hillary Clinton has spoken taken every opportunity recently to speak of about her religious roots, Barack Obama has been most comfortable talking about faith. The title of his political testament, The Audacity of Hope, was taken from a sermon by the pastor of his church.

In speeches he orates on the centrality of religion to sustaining progressive politics. “Imagine Lincoln's second inaugural addess without reference to the judgements of the lord. Or King's I Have A Dream speech without references to ‘all of God's children'. Their summoning of a higher truth helped inspire what had seems impossible, and to move this nation to embrace a common destiny”. Although not an evangelical, he is a longstanding friend of Wallis and has praised him and Campolo for their action on inequality.

Obama's call for change and renewal, itself a standard evangelical theme, has resonated. “I visit a lot of evangelical colleges and what I see is Obama stickers and Obama T-shirts all over the campuses,” says Campolo. “His is a voice that can inspire.” Although Hillary Clinton lacks the same capacity to excite, Campolo believes that many younger evangelicals consider her an acceptable alternative.

Both Wallis and Campolo are quick to distance themselves from the “Christian left”. Progressive evangelicals are hardly liberal on issues like abortion, but are anxious to “move away from endless symbolic legal debates which never change anything,” as Wallis puts it. He advocates instead practical methods of making abortion rarer, such as better support for low-income women. And he says that “the sanctity of life is a broader, deepers concern that includes Darfur, that includes the 30,000 children who will die today from hunger and disease.”

This attitude, says Campolo, is in tune with a younger generation of evangelicals. “Young people have a broader agenda and are interested in poverty and environmental issues. Gay marriage and abortion are at the bottom of the list. They are conservative on them but they have have gay friends and don't want to start a crusade against them.”

It does seem that evangelicals under 30 are deserting the Republicans - a Pew survey in the autumn revealed that this group gave President Bush only 45 per cent approval rating - down from a whopping 87 per cent in 2002. Although this has yet to greatly benefit the Democrats, it has meant a big rise in the number of uncommitted evangelical voters. Given that 78 per cent of evangelicals voted Republican in 2004, this could be crucial. Wallis estimates that half of evangelical votes are in play. “If democrats can win 40 per cent of evangelicals, that's the election,” he says.

John C. Green, Senior Fellow in Religion and American Politics at the Pew Forum on Religion and Public Life in Washington, agrees that younger evangelicals are increasingly concerned with “new issues”, like Aids and the environment, and that their traditional leaders are not speaking to these concerns. “There is a good bit of suppport in evangelical circles for these issues right now,” he says.

Democrats should not necessarily break out the champagne corks yet. Green says that “It is one thing to put ideas out there and get them debated, its another to get people to vote on these issues.” And if younger evangelicals are looking for a candidate who takes human rights and climate change seriously, while remaining quietly conservative on abortion, many of them will have to go no further than John McCain.

But if both parties feel the need to broaden their message to appeal to these voters, then Wallis's ideal of a strong “moral centre” could draw the partisan sting from Christian concerns. If progressive evangelicals can become a significant force in American life, then the era of the religious right might be drawing to a close.