Christian radio stations are broadcasting adverts in which Barack Obama describes how he let Jesus in his life as he knelt before a cross in a Chicago church 23 years ago.

"I felt that I heard God's spirit beckoning me," the Democratic nominee says, "I submitted myself to his will, and dedicated myself to discovering his truth."

The adverts are being funded by an organisation called the Matthew 25 Network, named after a biblical passage in which Christ promises redemption for those who care for the least and the lost.

Although not formally linked to Mr Obama's campaign, the network is part of a concerted effort to prise off a chunk of a Christian vote that has long been regarded as the more or less exclusive preserve of Republican presidential candidates.

On Saturday, he wrapped up a week in which he had focused on faith, patriotism and service by telling thousands of black Methodists in St Louis: "I won't be fulfilling the Lord's will unless I'm doing the Lord's work". He set out about plans, first unveiled on Tuesday, to expand the federal subsidies of President Bush's faith-based programmes with a $500 million fund for sending disadvantaged children to religious summer camps.

"It is not part of a political strategy," he insisted. "I say it because I believe it - I've always believed it. This is the work we are called to do as Christians."

Mr Obama, who recently held a private meeting in Chicago with almost 40 evangelical and Catholic leaders, is also planning to launch the Joshua Generation Project to attract the votes of evangelicals. This group, like Matthew 25, takes its inspiration from the Bible by putting Mr Obama in the role of Joshua who did what Moses could not by leading his people into the Promised Land.

His strategists acknowledge many Christian voters will still oppose Mr Obama because of his support for abortion and gay marriage. But they also point out that Republican nominee John McCain is struggling to convince the Religious Right of his credentials. They say there is untapped support among those who care about poverty, the Iraq war, climate change and Darfur.

Shaun Casey, a religious adviser to the Obama campaign, says the "moral basket" is now wider than the narrow issues on which many Christians have voted over the past 30 years.

In 2004, Mr Bush took 78 per cent of the evangelical vote - which accounts for at least a quarter of the electorate - and a narrow majority of Catholics. Mr Casey predicts that if Mr Obama can move these margins even by a small fraction then he "will be the next president of the United States".

But he still has an uphill battle. When Mr Obama found his faith in 1985 it was thanks to the Rev Jeremiah Wright, whose sermons - both in style and substance - are anathema to most white Christians.

Although Mr Obama has since quit that church, his exotic background and internet smears have persuaded more than one in ten Americans that he is Muslim. Talking about faith at the very least reminds voters he is a committed Christian.

Social conservatives, of course, remain sceptical - even hostile - towards Mr Obama's "confused theology". James Dobson, the founder of Focus on the Family, last month accused him of deliberately distorting the Bible "to fit his own world view".

But it was a sign of the times that the Rev Kirbyjon Caldwell swiftly leapt to Mr Obama's defence.

He is best known as the pastor who introduced Mr Bush at the 2000 Republican convention and recently officiated over the wedding of the president's daughter, Jenna.

Times Recommends

• Baghdad fury at claim US spied on PM
• British envoy criticised over Swaziland party
• How perfect is Sarah Palin’s First Dude?