WASHINGTON (AP) – With little hope of success, Senate Republicans pledged to forge ahead with a vote on a constitutional amendment to ban same-sex marriages.

Democratic opponents said they had more than 40 votes against the amendment, which needs 67 of the 100 senators to pass. Judiciary Committee Chairman Orrin Hatch said “This is so serious, sociologically and in so many other ways, that we do need to vote on it on the floor, one way or another.”

Senate Democrats said the Republican Party is playing politics with the highly contentious issue, by forcing a vote in mid-July, right before the Democratic National Convention, where Sen. John Kerry will receive his party’s nomination to run for president.

“This is not about the sanctity of marriage. This is about preserving a Republican White House and Senate,” said Democratic Sen. Patrick Leahy.

Massachusetts Gov. Mitt Romney, whose state is the only one in the country to recognize same-sex marriages, urged the Senate Judiciary Committee to pass the amendment banning same-sex unions, even as the conservative who wrote a federal law denying recognition to such marriages said that law was sufficient.

“It is not possible for the issue to remain solely a Massachusetts issue, it must now be confronted on a national basis,” Romney said. He said same-sex marriage “may affect the development of children and thereby future society as a whole.”

Former Rep. Bob Barr, a Republican who is opposed to same-sex marriages, told the committee that the Constitution shouldn’t be used as a vehicle to strangle states rights. His remarks suggested that some of the strongest opposition to the proposed amendment may come from conservatives who abhor same-sex unions.

The testimony sparked some testy exchanges between Romney and Senate Democrats, including Sen. Edward M. Kennedy, who questioned how the governor could back a proposed Massachusetts constitutional amendment allowing same-sex civil unions, and also support the federal amendment, which he said would prohibit states from permitting such unions.

“How can you have it both ways?” snapped Kennedy.

Romney said he would oppose the federal amendment if it prohibited states from granting civil unions or providing benefits to gay and lesbian couples. But he said he didn’t think the federal amendment did that.

Romney’s testimony came on the same day that former Massachusetts Republican Gov. William Weld was delivering the homily at the gay wedding in Boston of his former college roommate.

House Majority Leader Tom DeLay predicted the Senate vote would fail, and then “it is incumbent on the House to actually pass an amendment to put pressure back on the Senate to have a vote again.”

The issue has flared since last November when the Massachusetts high court ruled that prohibiting same-sex couples from marrying violates the state’s constitution. The Massachusetts legislature voted in March to amend the state constitution to ban same-sex marriages but allow civil unions.

Senate Majority Leader Bill Frist has said senators will begin debate July 12 on the proposed federal amendment. It states that “marriage in the United States shall consist only of the union of a man and a woman. Neither this Constitution, nor the constitution of any state, 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.”