WASHINGTON (Reuters) - A White House-backed drive to amend the U.S. Constitution to ban same-sex marriage headed toward an election-year defeat on Capitol Hill on Wednesday.

While two-thirds of the 100-member Senate must approve a constitutional amendment, Republicans scrambled to get a simple majority. And they admitted they did not have the 60 votes needed to survive a procedural vote set for Wednesday.

Republicans, accused by Democrats of pushing the measure merely to rally their socially conservative base for the November elections, vowed to try again -- perhaps next year.

"This issue is not going away," said Senate Majority Leader Bill Frist, a Tennessee Republican.

President Bush in February called on Congress to approve an amendment after Massachusetts' highest court ruled gay couples had a right to wed and San Francisco began issuing marriage licenses to same-sex couples.

Republicans contend gay marriage devalues traditional marriage, which they say is a pillar of civilization, and should be outlawed for the sake of children.

"I don't know of a more important debate in our country's history," said Sen. Orrin Hatch, a Utah Republican, in blasting "activist judges" for permitting same-sex marriages.

Republicans forced consideration of the proposed amendment two weeks before John Kerry, a U.S. senator from Massachusetts, will receive his party's presidential nomination at the Democratic National Convention in Boston.

Kerry and his running mate, Sen. John Edwards of North Carolina, oppose same sex-marriage, but argue amending the Constitution is not the answer. Like other Democrats, they say it should be left up to the states to define marriage.

Polls show most Americans oppose same-sex marriage, but are somewhat split on whether a constitutional ban is needed. Surveys also find voters believe many other issues are more important -- such as health care and national security.

Sen. Edward Kennedy, a Massachusetts Democrat, accused Republicans of bringing up the proposal for "pure politics."