The U.S. Senate (search) voted Wednesday against moving on a proposed constitutional amendment that would define marriage as a union between one man and one woman.

Bracing for defeat on one of President Bush's campaign-season priorities, Republicans vowed earlier Wednesday that a Senate setback would not halt their drive to enact the amendment.

The amendment proposal comes on the heels of a Massachusetts high court ruling allowing same-sex couples to marry and subsequent decisions by various cities and towns to perform gay marriage (search) ceremonies.

Measures involving hot-button issues often don't have enough votes when they first come up for a vote, whether it be at the federal or state level, noted Sen. Sam Brownback, R-Kan.

But "then the people look at it and say, 'wait a minute, I think a marriage should be between a man and a woman' … and they pass it" on the state level, Brownback told FOX News Wednesday before the procedural vote on the amendment. "So this is not an unusual scenario we're in."

"It really takes the American public a long time to deal with, and it should," he continued. "It's an important issue and I think you'll see this issue around for a long period of time."

The Senate GOP needed 60 votes to end debate on the measure before the actual amendment could even be voted on. The final vote on the proposal to vote on the amendment was 48-50 — far from the two-thirds majority needed to continue debate. Six Republicans joined dozens of Democrats in virtually defeating the amendment.

The Defense of Marriage Act of 1996 (search) currently doesn't bar states from legalizing gay marriages within their borders, but they're not obligated to recognize gay marriages performed in other states.

It also explicitly defines "marriage" as a "legal union of one man and one woman as husband and wife" and states that "spouse" refers only to a person of the opposite sex who is a husband or a wife.

"What is overlooked by many is that [the Defense of Marriage Act] has never been challenged in court successfully. Not once. That is the law of the land," said Senate Minority Leader Tom Daschle, D-S.D., noting that in 217 years, the Constitution has only been amended 17 times, although there have been 11,000 separate attempts.

"Is there some urgent need now … to amend the United States Constitution?" Daschle continued. "We have differences of opinion but there can be no difference in opinion in regards to how extraordinary a step there is  … this fundamental responsibility lies with the states, it has for two centuries."

But Sen. Orrin Hatch, R-Utah, said it's "only a matter of time" before the Defense of Marriage Act will be struck down, and that's cause for alarm and quick action. The Massachusetts high court has already said it's OK for gay couples to marry there.

"We are simply trying to preserve more than a 5,000-year old institution," Hatch said before the floor vote. "The most fundamental of all of our society that a few activist judges are trying to radically change … if such a foundation as this is not deserving of our protection … then I don't know what is."

An Election-Year Hot Potato

There were signs that supporters of the amendment intended to use it in the campaign already unfolding.

Senate Democrats had argued that politics prompted Bush and fellow Republicans to advance the issue to the top of the legislative agenda.

"We have something else going on here ... none of the various proposed constitutional amendments have gone through the judicial process to help the Senate determine whether a proposed amendment is necessary … changing the fundamental charter of our nation should not be proposed in a haphazard manner," Sen. Pat Leahy, D-Vt., said on the Senate floor Wednesday.

"We all know what this is ... it's a political exercise being carried out on the fly," Leahy continued. "Those trying to make this an election year issue see nothing out of bounds ... not even the Constitution."

Republicans protested, saying they are merely trying to preserve the sanctity of marriage.

The amendment "is to provide moms and dads for the next generation of our children. Isn't that important? Isn't that the ultimate homeland security — standing up, defending marriage, defending the right for children to have moms and dads, to be raised in a nurturing and loving environment? Isn't that what this debate is all about?" asked Sen. Rick Santorum, R-Pa., on the Senate floor Wednesday.

"I would ask them this question: What harm would this amendment … that simply restates the law of every state in this country and protects them from judicial tyranny … do?"

"It's not about hate, it's not about gay bashing," Santorum continued. "It's simply about what's doing the right thing for the basic glue that holds society together."

At issue is an amendment providing that marriage within the United States "shall consist only of a man and a woman."

A second sentence says that neither the federal nor any state constitution "shall be construed to require that marriage or the legal incidents thereof be conferred upon any union other than the union of a man and a woman." Some critics argue that the effect of that provision would be to ban civil unions, and its inclusion in the amendment complicated efforts by GOP leaders to gain support from wavering Republicans.

But gay-marriage supporters say Congress shouldn't even be playing such a role in the debate.

"Marriage has always been left to the states to regulate and govern within the context of the federal Constitution — we don't begin the process with asking Congress to change the Constitution," Evan Wolfson, author of "Why Marriage Matters," told FOX News.

Bush urged the Republican-controlled Congress last February to approve a constitutional amendment, saying it was needed to stop judges from changing the definition of the "most enduring human institution."

The Kerry, Edwards Factor

Democratic presidential hopeful John Kerry and his vice presidential running mate, John Edwards — both current U.S. senators —  released statements saying that had the amendment reached a vote, they would have voted against it.

Back on the campaign trail in Des Moines, Iowa, Wednesday, Edwards also said: "The Constitution should never be used as a political tool to divide Americans."

"The floor of the United States Senate should only be used for the common good, not issues designed to divide us for political purposes," Kerry added after the vote. "Even Republicans concede that this amendment is being offered only for political gains. The unfortunate result is that the important work of the American people — funding our homeland security needs, creating new and better jobs, and raising the minimum wage — is not getting done."

Edwards was the guest speaker at the House Democrat's weekly caucus Wednesday but didn't stay for the vote.

When asked if Edwards should have cast his vote, House Minority Leader Nancy Pelosi, D-Calif., said the vote "is a procedural vote one where the vote of Senator Edwards will not make a difference."

"I think Senator Edwards should be taking his voice to the American people as to why we need change in this country," she added.

On the issue of Congress' taking time to debate same-sex marriages, Pelosi said that regardless of how members feel on the issue, Congress is "wasting the public's time when we have important business to deal with."

The House Judiciary Committee on Wednesday was debating a measure to strip federal courts of jurisdiction over a portion of the Defense of Marriage Act; the full House may consider the legislation next week.

Other bills have been under consideration, although officials said it was unclear whether they would seek a vote on one of them. GOP aides said the leadership might schedule a vote on the House floor on a constitutional amendment closer to the elections.

FOX News' Molly Hooper and The Associated Press contributed to this report.