US President George W Bush has called for a constitutional amendment to ban same-sex marriages.
Mr Bush criticised rulings on the issue by "activist courts"
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.
He said the measure was needed because "activist courts" left no alternative.
An amendment stands little chance of being passed but analysts say Republicans see the issue as a vote winner in November's mid-term polls.
They say the president is seeking to switch the spotlight onto positive issues for his party in the wake of his slumping popularity - particularly over Iraq.
Mr Bush said: "Ages of experience have taught us that the commitment of a husband and a wife to love and to serve one another promotes the welfare of children and the stability of society."
Judges have recently rejected laws on traditional marriages in states such as Washington, California and New York.
"An amendment to the constitution is necessary because activist courts have left our nation with no other choice," Mr Bush said.
However, an amendment would require two-thirds support in the House and Senate. A similar measure was defeated in 2004.
Republicans are hoping their stance on an issue that opinion polls suggest is shared by the public will boost votes in November, when they fear losing control of Congress.
A recent Gallup poll suggested 59% of Americans were opposed to validating gay marriage in law.
The Democrats accuse their rivals of applying a smokescreen.
Senate Minority Leader Harry Reid said: "Bush Republicans would rather focus on purely divisive manoeuvres than real solutions that address the growing energy crisis."