BOSTON — Supporters of same-sex marriage in Massachusetts prevailed in a battle over a proposed constitutional amendment banning the practice, convincing enough state lawmakers to vote against it.

The Legislature in a special session yesterday voted 151–45 against the ballot measure seeking to define marriage in the state as the union between one man and one woman. More than 9,000 gay and lesbian couples have married since Massachusetts became the only U.S. state to permit the practice in 2003.

“In Massachusetts today the freedom to marry is secure,” Governor Deval Patrick said to cheers in the State House in Boston after the vote. “The good news beyond the merits of this vote is that we’ve put this issue behind us.”

Opponents gathered 170,000 signatures in 2005 backing the Massachusetts Protection of Marriage Amendment, which would have asked voters in a November 2008 ballot question to amend the state constitution to define marriage as between “one man and one woman.”

The amendment needed 50 votes from the General Assembly’s 200 lawmakers in two consecutive sessions to appear on the statewide ballot. It got 62 votes in January. Legislative leaders in May delayed a vote on the matter because they didn’t have enough support to defeat it.

“We’re going to look very closely at the circumstance by which people switched their votes,” said Lisa Barstow, a spokeswoman for VoteOnMarriage.org, the group formed in 2005 to advocate for a ban on the practice. “It was inevitable this was going to move to the ballot; anything else has to be suspect.”

Of the 24 Republicans in the Democratic-controlled Legislature, 17 voted to put the amendment on the ballot. Massachusetts legislators have grappled with same-sex marriage for years, killing another citizens’ initiative as well as an amendment lawmakers proposed limiting the practice.

Mr. Patrick, a Democrat who took office in January, said he and legislative leaders spent the last few weeks lobbying lawmakers to defeat the latest measure.

“This is huge,” said Arline Isaacson, co-chair of the Massachusetts Gay and Lesbian Political Caucus. “We’ve kept it off in 2004 and 2006 and now the 2008 ballot. They can’t keep this up. It becomes increasingly pointless.”

The Massachusetts Supreme Judicial Court in a 4–3 ruling in 2003 found the state couldn’t “deny the protections, benefits, and obligations conferred by civil marriage to two individuals of the same sex who wish to marry.” The case was Goodridge v. Department of Public Health.

Vermont, New Jersey, California, and five other states have extended some marriage benefits through civil union and domestic partnership laws.

New Hampshire this year passed such a law. Rhode Island, which hasn’t addressed the issue through its legislature or courts, has recognized same-sex marriages between its residents done in Massachusetts.

The federal government banned same-sex marriages through the 1996 Defense of Marriage Act, denying benefits such as spousal and survivor payments through Social Security. Many states have passed laws restricting either same-sex marriage or civil unions and domestic partnerships.