A constitutional amendment to ban gay marriage is headed toward certain Senate defeat, but supporters say new votes for the measure represent progress that gives the GOP's base reason to vote on Election Day.

"There's many of us who have not had an opportunity to debate and discuss this," said Sen. Mel Martinez, R-Fla., one of five freshmen supporters of the amendment who replaced opponents of the measure in the 2004 election.

Supporters say the amendment will win as many as seven new votes from freshmen elected after the amendment received its last vote in 2004. Their support is expected to produce a majority for the amendment in the 100-member chamber.

But 60 votes would be required for the measure to survive a test vote Wednesday and a two-thirds majority is required in both houses of Congress to send an amendment to the states. It then would have to be ratified by at least 38 state legislatures.

Still, supporters were pleased.

"We're building votes," said Sen. David Vitter, R-La., another new supporter. "That's often what's required over several years to get there, particularly to a two-thirds vote."

A majority of Americans define marriage as a union of a man and a woman, as does the amendment, according to a new ABC News poll. But just as many oppose amending the Constitution, the poll found.

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

"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 federal 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 vote on the amendment 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."

Like the Senate, the House in 2004 fell short of the two-thirds vote needed.

Bush, his popularity sagging and his conservative base dissatisfied with Republicans' efforts on social issues, issued a fresh appeal for passage Tuesday, 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.

Senate Democrats, all of whom except Sen. Ben Nelson of Nebraska oppose the amendment, say the debate is a divisive political ploy.

"The Republican leadership is asking us to spend time writing bigotry into the Constitution," said Sen. Edward M. Kennedy of Massachusetts, whose state 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."

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