Gay marriage opponents launched an initiative petition in Massachusetts this month that could put a constitutional amendment banning gay marriage on the state ballot by 2008.

The amendment would define marriage as the union between one man and one woman and would not make any mention of civil unions. It will likely sap conservative support away from an existing compromise amendment that would ban same-sex marriage while affirming partners’ rights to civil unions.

Massachusetts Governor W. Mitt Romney, who had previously supported the compromise, publicly endorsed this new proposal, calling it “a very clean, straightforward, unambiguous amendment.”

Boston Archbishop Sean O’Malley and the three Roman Catholic bishops in the state are also supporting the drive to collect signatures for the petition.

To qualify the petition for the state ballot, sponsor VoteOnMarriage.org must collect 65,825 signatures, after which the 32-word petition would need approval by only one-quarter of the Legislature, or 50 of its 200 members. Writing the amendment into the state’s constitution would require a simple majority at the polls.

A March 2005 poll by the Boston Globe found that a slight majority—56 percent—of Massachusetts voters supported gay marriage.

The compromise measure—the Travaglini-Lees amendment—cleared the legislature by a 105-92 vote last year, but would require reconfirmation by a majority of the 200-seat legislature this fall before it could reach the popular ballot in 2006.

Because the new proposal, unlike Travaglini-Lees, originated outside the legislature, it would require a lower threshold of support to reach the popular ballot: only one-quarter of the legislature needs to support the motion in consecutive sessions for it to pass through to the ballot.

If the amendment passed the ballot in 2008, it would nullify a November 2003 state Supreme Court decision that extended marriage rights to same-sex couples.

Backers of the amendment said that if the amendment passes, they will not seek to nullify the marriages that had already taken place. Approximately 6,000 same-sex couples have married in Massachusetts since November 2003.

By keeping the issue off the ballot until 2008, the VoteOnMarriage.org petition would extend the window for gay marriage in the state.

“We’re amazed they are considering letting us get married for two more years through 2008. It will virtually guarantee that many thousands more gay people will get married,” Massachusetts Gay and Lesbian Political Caucus co-chair Arline Isaacson told The Boston Globe.

The probably defunct Travaglini-Lees amendment was designed to satisfy both gay-marriage opponents and gay-union advocates, but it ultimately failed to please either camp.

“The Travaglini-Lees amendment is something that neither side wanted to begin with and still doesn’t,” said Massachusetts Family Institute Director of Public Policy Evelyn Reilly. “The proponents of same-sex marriages don’t want to define marriage as between a man and a woman, and the proponents of traditional marriage don’t want to put civil unions in the constitution.”

Romney’s shift in support away from the compromise has drawn the ire of same-sex marriage supporters, some of whom accuse Romney of pandering to conservatives outside the state to further possible presidential aspirations.

“Governor Romney is clearly focused on national ambitions, and I think it’s especially disturbing...that he is demeaning gays and lesbians to appeal to a national ballot,” said MassEquality Political Director Mark Solomon.

Supporters of the new amendment have also backed reciprocal benefits legislation, which would allow same-sex partners some of the privileges of marriage, including hospital visitation, on the basis of dependence rather than legal union.

But Solomon found these promises to be little consolation for gays and lesbians in light of the attempts to revoke gay couples’ marriage rights.

“The groups that are advancing this measure are some of the most anti-gay, right-wing groups in the country,” said MassEquality Political Director Mark Solomon. “These are groups that want to take away marriage equality and replace it with absolutely nothing. They want to take away every right.”

—Staff writer Samuel C. Scott can be reached at sscott@fas.harvard.edu.