A leading conservative evangelical on Tuesday said Democratic presidential candidate Barack Obama had distorted the Bible and espouses a "fruitcake" approach to the US Constitution.

The comments by broadcaster James Dobson are among the sharpest religious attacks to date on the Illinois senator, who will face Republican John McCain in the November election.

Dobson, who has previously said he will not vote for McCain because of his past support for stem cell research, on Tuesday said the Arizona senator was not doing enough to stop gay marriage in his home state.

"This is a year when we have a lot of frustration with both political parties," Dobson said on his radio show, which reaches millions of conservative listeners.

The criticism by Dobson, head of the Focus on the Family organisation and a strong backer of President George W Bush in 2004, comes as Democrats are hoping to make inroads among evangelical voters who have been a pillar of Republican support.

Speaking on his campaign plane during a tour of western states, Obama said Dobson's comments, which focused on a speech the Illinois senator gave in 2006, were a misreading of his words and amounted to "making stuff up".

In the speech, Obama said religious people do not have a monopoly on morality and should couch their arguments in universal, rather than religious terms.

"That is a fruitcake interpretation of the Constitution," Dobson said. "What he's trying to say here is, unless everybody agrees, we have no right to fight for what we believe."

Obama said in the speech that certain passages of the Bible, if interpreted literally, could allow parents to stone their children and require that the Defense Department be abolished.

"He's deliberately distorting the traditional understanding of the Bible to fit his own world view, his own confused theology," Dobson said.

OBAMA RESPONDS

But Obama said the speech was in no way an attempt to interpret the Bible.

"It is a speech that affirms the role of faith in, not just my life, but the life of the American people, that suggests that we make a mistake by trying to push faith out of the public square," Obama said.

"I do make the argument that it's important for those who, like myself, for whom faith is important, that we try to translate some of our concerns into a universal language so that we can have an open and vigorous debate, rather than having religion divide us," he said. "There's no theological work being done in that speech."

A fair reading of the speech shows that Obama is committed to working across religious lines, Joshua DuBois, the candidate's director of religious affairs, said in a statement.

Polls show evangelicals are slowly moving away from the Republican Party, though Obama's support of abortion rights and gay rights are likely to give pause to many.

Dobson has led efforts to outlaw abortion and gay marriage and helped get out the vote among evangelicals in Bush's 2004 re-election.

Democrats hope to win the support of younger, more centrist evangelicals who are concerned about global warming and poverty as well as abortion, the movement's traditional rallying point.

Obama, unlike past Democratic candidates, speaks frequently about his Christian faith.