'By this selfish action, Cheney is not merely disrupting society, she is being cruel to her child,' said Paul Cameron, of the Family Research Institute.

'Mary's pregnancy is an in-your-face action countering the Bush Administration's pro-family, pro-marriage and pro-life policies. She continues to repudiate the work to which her father has devoted his life,' said Janice Shaw Crouse, of the Concerned Women of America.

Ms Cheney, 37, resembles her father, sharing his squint and smirk. She also shares his dislike for the spotlight, granting only a handful of interviews despite the intense interest in her. She works near Washington, as chief of staff for the CEO of America Online, and has been with her partner, Heather Poe, for 15 years.

She came out to her parents as a teenager. She was so distraught over breaking up with her first girlfriend that she got into a car accident, and decided then to tell her parents. Her father reacted well, she said in a television interview this year.

'He's just this great even-keeled guy, and I told him and his reaction was, 'You know, look, you're my daughter and I love you, and I just want you to be happy.' And that was it,' she said.

But that private acceptance has not always translated into a political asset.

During the 2000 campaign, Mr Cheney's wife, Lynne, angrily denied during a radio interview that their daughter had publicly come out.

And during one of the 2004 presidential debates, Democratic candidate Senator John Kerry referred to Mary Cheney's sexuality when asked whether homosexuality was a choice. Lynne Cheney called it 'a cheap and tawdry political trick'. Neither parent made a comment when a Republican Senate candidate the same year called Mary Cheney a 'selfish hedonist'.

In the late 1990s, Mary Cheney worked as a gay and lesbian outreach co-ordinator for the Coors brewing company, whose anti-gay policy had made it the target of boycotts for decades. It worked and the boycott has been abandoned.

But she has not tried a similar outreach to get gays to vote Republican - President George W. Bush got 23 per cent of the gay vote in 2004, the same as four years earlier. She told Time magazine that the role her sexual orientation might play was not discussed in the 2004 campaign, in which she was chief of vice-presidential operations for the Bush-Cheney campaign, saying: 'We never had a conversation: Is this a positive? Is this a negative? It's not a political tool. It's my life.'

In the last few years, an increasing number of liberal states have begun to allow gay marriage or some sort of civil union for gay couples. Conservative states have responded by passing laws explicitly forbidding gays from marrying or allowing any sort of legal status for gay couples. This year, Republicans in Congress tried but failed to pass the Federal Marriage Amendment, which would have nullified all state efforts to allow gay unions.

And during the 2004 presidential race, several states had ballot initiatives to ban gay marriage, and Democrats said it was a tactic to rile up social conservatives and get them to the polls where they would also vote for Mr Bush. The president spoke out against legalising gay marriage, although Mr Cheney disagreed and said the matter should be left up to the states. Mary Cheney said later that she considered leaving the campaign over the issue and was targeted by gay advocacy groups for what they called her hypocrisy.

Mary Cheney says she considers herself and Ms Poe to be married, and will not divulge how she was impregnated. Virginia, where Ms Cheney and Ms Poe live, passed one of the nation's most-restrictive laws against gay unions last month, making it unclear what - if any - legal status Ms Poe will have with the baby.