A proposed constitutional amendment to ban same-sex marriage was headed for defeat in the Senate today, doomed by nearly solid Democratic opposition, sharp divisions within Republican ranks and a lack of consensus among voters over how best to deal with the issue.

Even with the strong backing of President Bush, the measure could have trouble attracting a simple majority of the Senate, GOP leaders acknowledge, let alone the two-thirds "super majority" needed to adopt a constitutional amendment. Yet GOP strategists hope the issue will help them in selected regions, and with crucial conservative voters, this fall.

While most senators are on the record against same-sex marriage, many in both parties are reluctant to amend the Constitution for anything, especially to override state prerogatives on a divisive social issue. Some also worried that the amendment would be viewed as gay bashing by middle-of-the-road swing voters. Still others said voters want Congress to deal with issues such as the economy, health care and Iraq, rather than to spend time in a losing battle over marriage.

"It's a difficult issue," said Sen. Robert F. Bennett (Utah), chief deputy whip for Senate Republicans. There is a "widespread feeling among some Democrats as well as Republicans that traditional marriage is under attack," he said, but members "don't want to be seen as gay bashers."

Many Republicans and their allies among Christian groups said they hope to turn a certain legislative loss into a political gain in this fall's elections by using the issue to mobilize conservative voters to turn out for Bush and congressional candidates in critical races.

Under this win-by-losing strategy, GOP leaders hope to reassure

conservatives that the party stands with them without angering moderates who are reluctant to amend the Constitution or target gays. Use of the issue would be confined to areas where it would do the most good.

"This is just the beginning of the process," said Gary Cass, of the Center for Reclaiming America, an advocacy group founded by the Rev. D. James Kennedy, a Florida evangelist. "We need to know who's with us and who's against us."

Republicans appeared to be laying down a political marker in scheduling the vote to occur less than two weeks before the Democratic National Convention, casting a spotlight on the Democratic presidential ticket's opposition to the amendment. Sen. John F. Kerry (Mass.) and his running mate, Sen. John Edwards (N.C.), said earlier they planned to return if there were an up-or-down vote on the amendment. But after it became clear that the showdown would be over a procedural issue, the two Democrats canceled out.

At issue in today's vote is whether to cut off debate on the proposed amendment, which defines marriage as existing only between a man and a woman. The proposed amendment also includes language that some have interpreted to cast legal doubt on civil unions.

Plans for a vote on the amendment itself collapsed after Republicans insisted on offering a scaled-back alternative, limited to defining marriage, and Democrats balked.

A two-thirds majority (67 votes if all senators are present) is required to amend the Constitution, and 60 votes are needed to cut off debate. It was not clear yesterday whether amendment sponsors could muster even a simple majority.

"I'm hoping we get 45 . . . maybe better," said Senate Republican Conference Chairman Rick Santorum (Pa.), a leader in the fight for the amendment.

Bush has recently stepped up his push for the amendment, mentioning it in a campaign appearance Friday and devoting his radio address to it Saturday. One GOP lawmaker said support for the constitutional ban may help Bush and other Republican candidates in rural sections of swing states such as Michigan, Pennsylvania, Ohio and Missouri. If these rural areas vote more heavily than urban areas -- as they often do -- then the strategy of keeping the issue stirred up will benefit Republicans, the lawmaker said.

But opinion polls generally show that most Americans agree with Kerry and Edwards in opposing same-sex marriage and a constitutional amendment to bar it, and some strategists question whether Bush and other Republicans are taking a risk in pushing the amendment.

As the battle for control of Congress has tightened in recent months, Republicans are looking to the marriage issue to boost their prospects in some critical races in the South.

But Democrats in many of these races -- Senate races in the Carolinas, Louisiana and Oklahoma -- have moved to neutralize the issue by embracing the proposed amendment, at least to a degree.

Political analysts say the controversy is likely to play little, if any, role in many other states, in part because a heavy emphasis on it could energize liberals as well as conservatives and antagonize swing voters. In some states, Republicans have had little to say about same-sex marriage.

The issue has received a heavy focus only in South Dakota, where former representative John Thune (R) used it recently in news conferences and radio ads in his race against Senate Minority Leader Thomas A. Daschle (D-S.D.), who opposes the amendment.

In South Carolina, Democrat Inez Tenenbaum moved early to defuse the issue by endorsing the amendment, as did Reps. Brad Carson (D) in Oklahoma and Chris John (D) in Louisiana. Erskine B. Bowles (D) in North Carolina has said he would support the amendment as a last resort, according to his campaign.

But Democrats cannot escape the issue by endorsing the amendment because they are running on the same ticket as Kerry and Edwards, said Dan Allen, spokesman for the National Republican Senatorial Committee.

The issue may loom larger in some House races involving incumbents who vote against the amendment, according to political analyst Stuart Rothenberg and others.

Outside groups have used it to energize supporters and solicit contributions.

The Human Rights Campaign, the nation's largest gay rights group, has spent about $1 million on advertising and an additional $1 million on grass-roots lobbying against the amendment since March, according to spokesman Steven Fisher.

Leading the charge on the Christian right is the Family Research Council. It has helped to collect about 2.5 million signatures on petitions against same-sex marriage and organized broadcasts in which famous evangelists have urged conservative Christians to contact their senators.

Staff writer Charles Babington contributed to this report.