President George W Bush has again urged the US Senate to pass a constitutional amendment banning same-sex marriage when it debates the issue this week.
Bush wants marriage defined as solely between a man and woman
Mr Bush said marriage between a man and a woman was the most fundamental institution of civilisation, and needed protection from "activist judges".
But opponents say the president is using the issue to win back disillusioned Republican voters.
The bill is widely expected to fail as it does not have the backing needed.
A two-thirds majority is required in both houses of Congress for an amendment process to begin.
The Senate is due to begin debating the amendment on Tuesday.
Mr Bush said an amendment would ensure no court could undermine what he said were the views of the American people.
"State legislatures are trying to address this issue, but across the country they are being thwarted by activist judges who are overturning the express will of their people," Mr Bush told reporters at the White House.
Judges have recently rejected laws on traditional marriages in states such as Washington, California and New York.
The BBC's James Coomarasamy in Washington says a slight majority of Americans are against same-sex unions - but most want their individual states to make the final decision about their legality.
Democrats accuse Mr Bush of cynically promoting an issue which appears to have little chance of Congressional approval to appeal to the Republicans' conservative base ahead of November mid-term elections.
Mr Bush has been experiencing slumping popularity in opinion polls - particularly over Iraq.
On Saturday Mr Bush used his weekly radio address to deliver a plea for the US Senate to formally define marriage as the union of man and woman.