The constitutional amendment to ban gay marriage is headed toward an unsuccesful end Wednesday despite a fresh appeal for passage by President Bush. A vote to wrap debate is expected in the late morning. It is all but certain to fail.

While a majority of Americans oppose gay marriage, an equal majority oppose amending the Constitution to prohibit it, according to a poll this week by ABC News. And, according to one Republican, Americans do not yet buy the warning that traditional marriage is under attack by renegade judges.

"Most Americans are not yet convinced that their elected representatives or the judiciary are likely to expand decisively the definition of marriage to include same sex couples," said Sen. John McCain, R-Ariz., a possible presidential candidate in 2008. He told the Senate on Tuesday he does not support the amendment.

The measure's looming defeat in the Senate is by no means its last stand, said its supporters.

"Whether it passes or not this time, I do not believe the sponsors are going to fall back and cry about it," said Sen. Orrin Hatch, R-Utah. "I think they are going to keep bringing it up."

The House plans a redux next month, said Majority Leader John Boehner, R-Ohio.

"This is an issue that is of significant importance to many Americans," Boehner told reporters. "We have significant numbers of our members who want a vote on this, so we are going to have a vote."

Some of those closely watching the Senate's three-day debate on the proposed amendment are engaging that argument as if the measure stands a chance of passage.

Bush, his popularity sagging and his conservative base dissatisfied with Republicans' efforts on social issues, issued a fresh appeal for passage Tuesday for the third time in as many days.

"The administration believes that the future of marriage in America should be decided through the democratic constitutional amendment process, rather than by the court orders of a few," a White House statement said.

The Vatican also weighed in Tuesday, naming gay marriage as one of the factors threatening the traditional family as never before.

Despite the big names advocating it, the amendment was expected to fail a test vote early Wednesday after three days of floor time in which debate over the definition of marriage touched on issues ranging from child welfare to bigotry.

Democrats, all of whom except Sen. Ben Nelson of Nebraska oppose the amendment, say the debate is a divisive effort to energize social conservatives this election year.

"The Republican leadership is asking us to spend time writing bigotry into the Constitution," said Sen. Edward Kennedy of Massachusetts, which legalized gay marriage in 2003. "A vote for it is a vote against civil unions, against domestic partnership, against all other efforts for states to treat gays and lesbians fairly under the law."

More than half of the 100-member Senate is likely to support the amendment on the test vote Wednesday, even if the vote fails to win the required 60 votes, said Hatch.

"Does he really want to suggest that over half of the United States Senate is a crew of bigots?" he fumed.

More than half of Americans, 58 percent, said in an ABC News poll released Monday that same-sex marriages should be illegal. But only 40 percent said they support amending the Constitution to ban gay marriage. A majority said states should make their own laws on gay marriage.

Forty-five of the 50 states have acted to define traditional marriage in ways that would ban same-sex marriage — 19 with constitutional amendments and 26 with statutes.

The amendment would prohibit states from recognizing same-sex marriages. To become law, it would need two-thirds support in the Senate and House, and then would have to be ratified by at least 38 state legislatures.