The U.S. Senate Monday opened debate on a proposed Constitutional amendment to ban gay marriage in what is seen as an appeal by Senate Republican leaders to their party's conservative base this congressional election year. But the measure lacks the necessary support to survive a vote later this week.

The proposed Constitutional amendment has the support of President Bush, who called on the Senate to act on the measure out of concern that judges in several states have overturned laws defining marriage as between a man and woman.

"Marriage is the most fundamental institution of civilization, and it should not be redefined by activist judges," said President Bush.

Senator Wayne Allard, a Colorado Republican who is a sponsor of the proposed amendment, agrees.

"Activists and lawyers have devised a strategy to use the courts to redefine marriage," said Wayne Allard. "This strategy is a clear effort to override public opinion and the long-standing opposition to traditional marriage and to force same-sex marriage on society."

But critics say the only reason President Bush and Republican supporters are promoting the measure is to appease members of the party's conservative base, who - according to public opinion polls - have grown disaffected with the Bush White House and the Republican Congress over the war in Iraq, immigration reform and other issues.

Senator Patrick Leahy is a Vermont Democrat:

"The Republican leadership's strenuous efforts to move this proposed amendment to the Senate floor for debate shows how important it is to the Republican leadership of the Senate to cater to the extreme right wing and special interest groups agitating for a fight over this issue," said Patrick Leahy. "They intend to stir up an election year fight and use it as a campaign tool in a political strategy."

Many social conservatives applaud the push for a Constitutional amendment. But others see it as purely political.

Bruce Fein, former Associate Deputy Attorney General in the Reagan administration, questions why President Bush and Senate Republicans waited until five months before the congressional elections to address the issue.

"The political motivation behind this is simply not to advance a conservative political principle, but simply to advance the agenda that is essential to the Bush administration's popular support and the Republican Party's popular support going into the 2006 elections," said Bruce Fein. "That seems to me, when it comes to a matter of such high moment as amending the Constitution, to rank no higher than a very squalid motive."

The Senate is to vote on the measure later this week. But it faces stiff opposition from Democrats and Republican moderates, who believe the issue should be decided by the states.

A similar amendment failed in both the Senate and House of Representatives in 2004.

Public opinion polls show a majority of Americans oppose same-sex marriages, but that opposition has been eroding in recent years.