WASHINGTON - The US Senate has voted down a constitutional ban on same-sex marriage that had been backed by President George W. Bush.

The 49 to 48 Senate vote fell short of the 60 votes needed to clear a procedural hurdle, but Republicans plan a vote in the House of Representatives to keep a national spotlight on the issue.

President Bush has said the Constitution must be amended to prevent judges from striking down existing state bans on gay marriage.

However, Democrats accused Republicans of exploiting a divisive issue they knew would fail in order to shore up conservative support before November congressional elections.

"It is a cynical attempt to score political points by overriding state courts and intruding into individuals' private lives," Democratic Senator Edward Kennedy of Massachusetts said in floor debate on Tuesday.

The US House of Representatives is expected to take up the marriage amendment in July, though Republican leaders do not expect it to pass there either.

Constitutional amendments must win approval from two-thirds of each house of the US Congress and three-quarters of state legislatures before taking effect.

Colorado Republican Sen. Wayne Allard, the bill's sponsor, did not expect the gay-marriage ban to pass but hoped to demonstrate increased support since 2004, when 48 senators voted for a similar bill.

Allard and other backers said they were not disappointed that the measure only won 49 votes this time.

Seven Republicans voted against Allard's bill on Wednesday, including two - Pennsylvania Sen. Arlen Specter and New Hampshire Sen. Judd Gregg - who supported it in 2004.

Specter said he opposes same-sex marriage but would vote against the measure because it tramples on states' rights.

Two Democrats, Nebraska Sen. Ben Nelson and West Virginia Sen. Robert Byrd, voted for the gay-marriage ban.

According to a March 2006 poll by the Pew Research Center, 51 per cent of Americans oppose same-sex marriage, down from 63 per cent in February 2004.

Forty-five states have passed laws or amended their constitutions to prohibit same-sex marriage, and the 1996 Defence of Marriage Act allows states to refuse to recognise marriages performed elsewhere.

State judges have struck down several state gay-marriage bans and court challenges are pending in nine states.