WASHINGTON (CNN) -- The Senate was voting Wednesday morning on a bid to amend the Constitution to essentially ban same-sex marriage, even though supporters concede the measure does not have enough votes to pass. Nonetheless, proponents defended the decision by Republican leaders to bring the amendment to the Senate floor, dismissing complaints from opponents that the measure is an election-year ploy that wasted precious time on the legislative calendar. "I think this debate is very healthy, and it's winning a lot of hearts and minds. I think we're going to show real progress," Sen. David Vitter, a Louisiana Republican, said. "The federal marriage amendment debate simply is an opportunity for us to affirm our support for marriage," said Sen. John Thune, a South Dakota Republican. "It is an important debate to have in this country." But Sen. Ted Kennedy, a Massachusetts Democrat, denounced the proposed amendment Tuesday as "an instrument of bigotry and prejudice," which he said was designed by the GOP leadership "to try to bring Republican senators out of the ditch of disapproval." And Senate Minority Leader Harry Reid, a Nevada Democrat, said that "the reason the Senate Republicans are pushing this marriage amendment is because they don't want to address the real issues of this country." "This is an effort by the president and the majority in the House and the Senate to distort, to misdirect what the real issues are," he said Tuesday. The Senate on Wednesday morning was voting not on the amendment itself but on a procedural motion to cut off debate, which requires 60 votes to pass. The motion was expected to fail, effectively blocking the measure from clearing the Senate. (Watch the politics behind the proposed ban -- 4:32) The vote began around 10 a.m., after a final hour of debate, a GOP leadership source told CNN. The Senate began debate on the amendment Monday afternoon. Even if the measure were able to clear the procedural vote, a two-thirds majority -- 67 votes -- would be required for final approval of a constitutional amendment -- an even higher hurdle to overcome. However, supporters were predicting they will be able to eke out a simple majority in Wednesday's test vote, which they insist would be a sign of momentum. The last time the Senate voted on the amendment, in July 2004, only 48 senators supported it and 50 were opposed. Since then, however, five Democrats who voted against the measure have been replaced by Republicans who support it, including Vitter, Thune and Sens. Mel Martinez of Florida, Jim DeMint of South Carolina and Richard Burr of North Carolina. McCain opposition Sen. John McCain on Tuesday told the Senate that he would oppose the amendment despite his belief that "marriage should be reserved for the union of a man and a woman." "I disagree that the current Constitutional structure provides insufficient mechanisms for ensuring that the public meaning of marriage is not tampered with by activist judges," the Arizona Republican said, according to prepared remarks. Spurred on by religious conservatives in his political base, President Bush has called on the Senate to approve the amendment, saying it is necessary to protect the institution of marriage from state court decisions striking down marriage laws that exclude gay and lesbian couples. So far that has happened in just one state, Massachusetts, where same-sex marriages became legal in 2003, although court cases are pending in a number of other states. But proponents of the ban expressed a sense of urgency. "As we speak, there are nine states whose law is protecting the traditional definition of marriage are being challenged in court," Senate Majority Leader Bill Frist said on Monday. However, even some opponents of same-sex marriage are wary of the federal amendment, which they see as an unwarranted intrusion into an area traditionally left up to states. Martinez dismissed that argument at a news conference Tuesday, insisting that "it isn't good enough to say, 'Leave it up to the states.' " "If we leave it up to the states we will see the erosion of marriage that we've seen by activist courts, which we otherwise will not see if we protect the institution of marriage at the federal level," he said. To become part of the Constitution an amendment needs approval from at least two-thirds of the Senate (67 of the 100 members), at least two-thirds of the House (290 of the 435 members) and three-fourths of the states (38 of the 50 states), or by a convention called by three-fourths of the states. In the nearly 220 years since the Constitution was written, only 27 amendments have made it through this arduous approval process, the most recent in 1992 governing the timing of changes in congressional compensation. No amendment has been approved by a convention. Source Do you think that raising issues that are very controversial yet unlikely to pass is a good use of Congress's time? How do generally feel about the issues raised by this administration in the second term?