Cheney, who is politically close to her father, has always declined to be a spokeswoman for gay rights yet subtly and persistently she is altering the terms of the debate about the family.

One commentator in The Washington Post called her pregnancy an “Ellen DeGeneres moment”, a reference to the coming out of the television star in 1997, which made lesbianism acceptable in Hollywood.

Precisely how Cheney, 37, became pregnant is unkown. When the news emerged last week that she was expecting a baby with her girlfriend of 15 years, Heather Poe, some gay and lesbian groups initially carped about the hypocrisy of the Republican party and “grandfather Cheney” almost as much as pro-family organisations lamented the existence of “two mommies”.

In a change of tone, however, gay rights groups have gone on to praise Dick Cheney and his wife Lynne — an outspoken conservative and author of patriotic history books for children — for their enthusiastic comments about the pregnancy. Matt Foreman of the National Gay and Lesbian Task Force said he wanted to “thank” the Cheneys for saying they were “looking forward with eager anticipation’’ to the birth of their sixth grandchild.

Foreman grew up in Dick Cheney’s home state of Wyoming. “I never agreed with his policies but I always thought the vice-president would put his family above anything else. It really feels that way now. He’s standing up for his lesbian daughter and that’s admirable.”

Although some conservative groups have not shirked from criticising Cheney and Poe — “It’s tragic that a child has been conceived with the express purpose of denying it a father,” said Robert Knight, of the conservative Media Research Center — the Republican right has largely fallen silent.

Kathryn Jean Lopez, editor of the conservative journal National Review Online, said: “Unless Mary Cheney asks to be a spokeswoman on the issue, folks ought to leave her alone.”

The vice-president has previously made it clear he disagrees with the Bush administration’s opposition to same-sex marriage, saying people “ought to be free to enter into any kind”.

Cheney told her father she was a lesbian when she was a schoolgirl — on the day she crashed the family car. Her father’s reaction was much the same then as now: “You know, look, you’re my daughter and I love you and I just want you to be happy,” he said.

She went on to meet Poe — a “smart, warm, funny and incredibly private person” — when she was at university and they were on rival women’s hockey teams. They had managed to stay out of the spotlight for eight years, she recalled, when her father said he would run for vice-president. She knew then that their relationship would come under scrutiny.

Cheney was “furious” when John Edwards, the Democratic vice-presidential nominee in 2004, brought up her sexuality in a debate with her father.

The ensuing furore, however, tempted her to write her autobiography, Now It’s My Turn, and to speak up about her partnership with Poe.

“From our perspective we’re already married,” she said. “She’s the person I want to spend my life with.”

Cheney also went on to defend gay parenthood: “What matters is that children are raised in a loving and supportive environment and there’s no reason why that can’t be provided by a same-sex couple.”

The pregnancy has drawn attention to tough laws against gay marriage and civil unions in Virginia, where Cheney and Poe live. Only Cheney, the biological mother, will have “full legal status” as a parent and the state does not allow same-sex parents to adopt.

Virginia voters affirmed their opposition to gay marriage at the ballot box during last month’s mid-term elections.

Despite her regard for privacy, Cheney is likely to be pleased by the comparison of her pregnancy to DeGeneres’s coming out. After Edwards’s very public outing of her in 2004, America’s biggest talk show stars besieged her for an interview.

Their messages piled up on Cheney’s voicemail. “The only person I was tempted to call back was Ellen DeGeneres,” she recalled in her memoir. “I am a fan.”

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