BOSTON (AP) – A fragile coalition of lawmakers cobbled together last year to support an amendment seeking to ban same-sex marriage has collapsed, virtually guaranteeing same-sex marriage will remain legal in Massachusetts, at least for now.

A poll of lawmakers conducted by The Associated Press has found at least 104 who planned to vote against the amendment when it comes up for a second vote this week. That’s enough to defeat the measure, which would also create civil unions.

Last year, months after the Supreme Judicial Court ruled that same-sex marriage was legal under the state constitution, the measure passed by a 105-92 margin. To get on next year’s state ballot, the amendment needs the support of a majority – at least 101 – of the state’s 200 lawmakers in the second round of voting.

Opposition to the measure is likely even deeper than the survey indicates.

Several lawmakers who voted against the proposal last year could not be reached for comment. Others who have voiced strong opposition to the amendment in the past declined to respond to the survey.

The AP attempted to reach all 200 lawmakers with at least two phone calls between Sept. 6 and Sept. 9. Of those reached, 104 said they would vote against the proposal, 19 said they would support it, and 3 said they were undecided.

The reasons for the collapse are many, rooted in the language of the amendment, which seeks to broker a compromise between foes of same-sex marriage and supporters of gay rights by outlawing same-sex marriage but enshrining civil unions.

The compromise ultimately had an opposite effect, alienating foes of same-sex marriage by creating civil unions and offending gay rights supporters by banning same-sex marriage.

Perhaps the best indication that support for the amendment – which was already eroding – is in free-fall are the number of lawmakers abandoning their support of the measure.

More than a dozen lawmakers who voted for it the first time around said they will switch their votes this week, either because they fully support same-sex marriage or oppose civil unions.

Others simply said that after more than a year of watching gay and lesbian couples marry with no ill effect on society, they see no need to rescind the right.

Rep. Anne M. Gobi, D-Spencer, had a change of heart after seeing how the opportunity to marry has changed the lives of so many couples. She said she could not support the compromise amendment, as she did last year.

“I haven’t talked to any married heterosexual couples that have felt threatened by same-sex marriages,” she said. “When you look at the world situation and all the terrible things that are happening, there’s a lot worse things … than allowing two people who love each other to be together.”

Rep. James Brendan Leary, D-Worcester, said he didn’t want to use the state constitution to take away rights rather than create new ones.

“It’s a dangerous precedent to take away rights that have been granted by the court for an identifiable group of people,” he said. “It’s not simply a policy issue. It’s a question of how we use our constitution.”

Many foes of same-sex marriage, who supported the amendment in the hopes of preventing same-sex marriages from happening, are drawn to a second, much stricter alternative amendment that would ban same-sex marriage without granting civil unions.

The earliest that proposal, which cleared a key hurdle last week when it was certified by Attorney General Thomas Reilly, could go before voters is 2008.

“We are going back to the beginning and defining marriage as the union of one man and one woman,” said Rep. Philip Travis, D-Rehoboth, who voted for the compromise amendment last year, but now plans to vote against it.

Supporters of that amendment must still collect the signatures of 65,825 registered voters, and win approval for it in two sittings of the Legislature. But because the amendment begins with citizens, only a quarter of lawmakers – a much lower threshold – must vote for it.

Same-sex marriage opponents said they reluctantly voted for last year’s compromise and welcome the chance to vote for a simple ban.

“I was not a supporter of gay marriage. The [compromise] amendment was the only amendment at that time, because all the other amendments were defeated,” said Rep. Paul J. Donato of Medford, who said he would support the new proposal.

Former Boston Mayor Ray Flynn, a same-sex marriage opponent, said the new proposal is superior because it enshrines marriage between a man and a woman, which he said is best for children. But he added the measure would ensure rights for non-married couples, such as inheritance or hospital visitation rights. It’s worth the wait to vote on it, he said.

“I’d rather get it right than discriminate or hurt anybody,” he said. “I think that it strikes that fair balance.”

Some lawmakers who have supported same-sex marriage in the past declined to respond to the survey, saying they believed the vote was still too narrow.

Rep. Byron Rushing, D-Boston, a vocal backer of same-sex marriage whose district includes Boston’s heavily gay South End, declined to say if he would again vote against the same-sex marriage ban.

“Not on this issue,” he said, when asked how he would vote. “It’s too close.”