The U.S. Senate is expected to start debate on a proposed constitutional ban that would bar same-sex marriages.
The proposal, which is being sponsored by Colorado Senator Wayne Allard, would change the constitution to state that -- quote -- "Marriage in the United States shall consist only of the union of a man and a woman."
Allard, a Republican, says he wants to prevent judges from defining marriage without input from the majority of Americans.
Last month, the Senate Judiciary Committee approved the amendment.
But its chances of passing the full Senate are slim.
Democrats and opponents of the measure say Republican leaders are pushing the amendment in an election year purely for political reasons.
Allard and Colorado Representative Marilyn Musgrave, also a Republican, sponsored the amendment in 2004, but it failed in both the House and Senate.