Standing in a cavernous hall in the headquarters of the evangelical group Focus on the Family, Lisa Anderson has some bad news for John McCain. 'This is going to be an interesting election,' she said with a smile.

The phrase conveys a stark warning to the Republican candidate. Focus on the Family, where Anderson is a director, is one of the most powerful evangelical groups in America. Many of its supporters - and millions of other US evangelicals - helped elect George W Bush twice, giving him their votes and volunteering for his campaign. McCain, who desperately needs their votes, would like them to support him, too. But many evangelicals neither trust nor like McCain.

Nowhere illustrates McCain's problem with evangelical voters better than Colorado Springs, in the shadow of the Rocky Mountains. Dubbed 'the Vatican of the evangelical movement', the city is headquarters for more than 100 Christian organisations, ranging from mega-churches with 10,000-strong congregations to Focus on the Family, whose radio broadcasts reach 200 million listeners around the world each day.

Many evangelicals visit Focus's huge headquarters on holiday, inspired by its message of being against abortion, gay marriage and the morality of Hollywood. But one person who has not yet made the pilgrimage is McCain, for Focus's founder, James Dobson, has pointedly not endorsed him, despite backing Bush in 2004. That position is not likely to change. 'I don't think we will see the trend in that direction,' said Anderson.

McCain's problems with attracting evangelical votes are long-standing. In 2000, after defeat in South Carolina against Bush, he railed against evangelical leaders as 'agents of intolerance'. Unlike Bush, he makes few references to his personal faith. He has not supported banning stem cell research, but has promoted campaign finance reform that evangelicals oppose and attacked a federal law banning gay marriage.

Kay Parsons, a housewife shopping in a Colorado Springs mall, summed up evangelical attitudes: 'I don't really trust him.' Parsons, who voted Bush in 2004, is not sure she will vote for McCain. 'I am not happy with any of the choices in front of me,' she said.

The size of the evangelical community ensures its voice must be listened to. In the post room of Focus on the Family, dozens of workers sift through the mail, which can be as much as 150,000 items a day. Such power and influence mean evangelicals are a voting bloc McCain cannot write off. His campaign is bombarding 600 nationwide leaders with regular emails and appeals for help. Plans have been drawn up to mobilise the evangelical vote in 18 vital states. His top staff, like senior aide Charlie Black, have regular meetings with evangelical leaders. It might work. McCain's record on the key issue for many conservative evangelicals - abortion - is solidly hostile. 'The evangelical community will come around in the end,' said Steve Mitchell, a political pollster and chairman of Mitchell Research. 'Some leaders have not endorsed him yet because they are just tough negotiators. They are playing politics.'

Yet McCain's problems have led to speculation about a narrowing 'God gap' between Republicans and Democrats. Some of Barack Obama's aides believe they can move into the evangelical bloc and win over many of the voters that elected Bush. In Chicago last week Obama met 30 religious leaders, including the Rev TD Jakes, pastor of a Dallas mega-church. His staff also held the first fundraiser for Obama by a new group of evangelicals called the Matthew 25 Network.

Democrats may also benefit from profound changes in the evangelical community, estimated to comprise almost one in four voters. In recent years many churches and leaders have embraced issues such as poverty, Aids and environmentalism that differ from the usual Republican platform. In recent weeks TV viewers have been watching ads warning about climate change featuring the unlikely duo of the left-wing Rev Al Sharpton and rightwinger Pat Robertson. Recently more than 100 leading evangelicals issued a new manifesto, one of the key points of which called for a return to a more purely religious identity. The document was an implicit rejection of the evangelical activism effectively harnessed by the Republican party.

This national story has been mirrored in Colorado City. Its largest church is the New Life Church founded by the Rev Ted Haggard, a preacher who boasted about having weekly chats with Bush. New Life was a potent symbol of rising evangelical power in the White House. But Haggard's prominence collapsed following a gay sex scandal. Now New Life's minister is the Rev Brady Boyd, who has rebuilt the church on non-political grounds and has stated he will not suggest who his flock should vote for.

Many evangelical voters could be swing voters or even be prepared to go Democratic. But experts believe Obama also faces problems, especially after the scandal over the incendiary speeches of the Rev Jeremiah Wright, his former pastor. Wright's religious vision was born in America's black churches and is in a very different tradition from most evangelism. Mitchell believes the Wright debacle will ensure that McCain will win over the constituency. Nevertheless, McCain's problems still leave him walking a tightrope. He needs evangelical voters, but also needs to retain his appeal to the more moderate middle ground. McCain was recently forced to renounce the endorsements of two religious leaders because of their past extremist statements on Islam and Jews.

McCain may find that his balancing act between the Republican base and the middle ground becomes impossible. Take two Colorado City women: Susan Henderson and Cindy Smith. Both were Bush voters in 2004. Both distrust McCain. 'I am pro-choice and he's not,' said Henderson. Smith said: 'I'm the conservative type. McCain is a bit too much for the other side.' Neither would say she would definitely vote for McCain in November. Nor would they reject Obama outright.

As Anderson said, when it comes to the evangelical vote it is going to be an 'interesting' - and crucial - battle.