America's largest women's rights organisation delivered a snub to Sarah Palin's history-making candidacy yesterday by endorsing Barack Obama and Joe Biden's bid for power.

The National Organisation for Women (NOW) is 500,000 strong and hugely influential. The feminist organisation almost never supports a presidential candidate, but the Alaska governor's Christian fundamentalist faith and her opposition to abortion rights has forced its hand.

Other women's rights organisations are also campaigning against Governor Palin, pushed along by a spontaneous anti-Palin movement among women.

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In Alaska at the weekend, a Welcome Home rally for Mrs Palin was dwarfed by a demonstration organised by Alaska Women Reject Palin, which was held on the lawn of a downtown Anchorage library.

After triggering a huge surge of enthusiasm for John McCain's campaign and sending him into the lead over Barack Obama, Mrs Palin has come under intense scrutiny from the national media. The investigation known as "Troopergate" threatens to expose her as a bullying governor who fired Alaska's de facto police chief for family rather than professional reasons.

The controversy moved up a notch yesterday when Mrs Palin's campaign team announced her refusal to co-operate with the Troopergate investigation because it was "tainted" by politics.

NOW's decision to back Senator Obama when a woman is within striking distance of becoming elected is a bold step for the group and a setback for John McCain's hopes of luring the millions of women who supported Hillary Clinton in the Democratic primaries.

"The addition of Sarah Palin gave us a new sense of urgency," Kim Gandy, the head of NOW, told National Public Radio. "She is being portrayed as a supporter of women's rights... as a feminist when in fact her positions on so many of the issues are really anathema to ours.

"A lot of women think it's a great thing for a woman to be running for vice-president," she continued, "but they are completely dismayed when they find out her positions. The idea that she opposes abortion even in cases of rape and incest – those kinds of positions are completely out of step with American women and once they find out about those positions, they get a little less excited."

Last week, Alaska legislators ordered Mrs Palin's husband, Todd, as well as her chief of staff and deputy chief of staff to answer questions at the Troopergate inquiry. Several years ago, Governor Palin's sister went through a custody battle with her ex-husband Trooper Mike Wooten. Efforts to sack Trooper Wooten failed and Governor Palin sacked the safety director Walt Monegan instead. Mrs Palin's refusal to co-operate with the panel could raise more doubts her suitability as a national candidate in the minds of voters.

The governor's creationist beliefs have also emerged as a hurdle. In Wasilla, where she cut her teeth as a Republican mayor, the Rev Howard Bess, a Baptist minister and author of the book Pastor, I am Gay, says his work was on then Mayor Palin's "hit list" for removal from the town's library. "People in city government have confirmed to me what Sarah was trying to do," he said.

Another Wasilla resident, Phil Munger, a music composer and teacher, says she pushed an evangelical agenda in the town. "She wanted to get people who believed in creationism on the [school] board. I bumped into her after my band played at a graduation ceremony at the Assembly of God [a church]. I said, 'Sarah, how can you believe in creationism – your father's a science teacher.' And she said, 'We don't have to agree on everything.' I pushed her on the earth's creation, whether it was really less than 7,000 years old and whether dinosaurs and humans walked the earth at the same time. And she said yes, she'd seen images somewhere of dinosaur fossils with human footprints in them."

Mr Munger also asked Mrs Palin if she believed in the End of Days, the doomsday scenario when the Messiah will return. "She looked in my eyes and said, 'Yes, I think I will see Jesus come back to earth in my lifetime'."