Just a week after his wife, Laura Bush, said that same-sex marriage should not be used as a campaign tool, the President did just that, reigniting a hotly-contested cultural issue in what Democrats claimed was a craven attempt to appease and mobilise social conservatives.

Speaking in the White House before an audience of religious conservatives, community leaders and family organistions, Mr Bush said: “Marriage is the most fundamental institution of civilisation and it should not be redefined by activist judges,” a clear reference to the Massachusetts supreme court which in 2003 ruled gay marriage legal in the state.

“You are here because you strongly support the definition of marriage as a union between a man and a women. I am proud to stand with you.”

After backing a constitutional gay marriage ban as part of his re-election campaign, Mr Bush has barely mentioned it since, dismaying cultural conservatives who claim they have been largely abandoned by the White House since 2004.

Amid public disaffection over the Iraq war, political corruption and high petrol prices, and with Mr Bush’s approval ratings at a record low, Republicans fear losing Congress this November, making high turnout among the party’s base crucial to blocking a Democrat takeover.

In recent weeks Karl Rove, Mr Bush’s chief political strategist, has made clear to the President how disaffected religious conservatives have become — Mr Bush’s support among conservatives has gone from 91 per cent to 68 per cent — and relayed their threats to stay at home in November unless their agenda is implemented.

Mr Bush’s call for a gay- marriage amendment coincided with the start of a full Senate debate on the issue, led by the Republican leadership there.

Bill Frist, the Republican Senate leader, has also scheduled a debate on a constitutional ban on burning the US flag, a favourite conservative issue.

Neither the gay marriage nor flag burning amendments have any hope of gaining the two-thirds support needed to win Senate passage, let alone the support of three quarters of the states that is also needed to amend the US Constitution.

Predictably, Democrats derided the move. Joe Biden, a senator with his eyes on the 2008 presidential race, said: “The world’s going to Hades in a handbasket. And we’re going to debate in the next three weeks gay marriage, a flag amendment and God only knows what else. I can’t believe the American people can’t see through this.”

The move also proves awkward for Dick Cheney, the Vice-President. His daughter Mary, a Republican and a lesbian, published a book last month in which she expressed her distaste for Mr Bush’s opposition to gay marriage in 2004. Mr Cheney says he does not support a constitutional ban.

Times Recommends

• The Japanese Prime Minister is doomed: or is he?
• Brighton company 'involved in cyber attack'
• The art of survival: it's all in the mind