BOSTON (AP) – Same-sex marriage opponents filed a ballot initiative with the state on Aug. 2 that, if passed, would amend the state constitution to ban same-sex marriages.

The move, which was expected, casts doubt on the future of another ballot initiative pending in front of the Legislature.

The Massachusetts Family Institute submitted the citizen’s petition a day before the deadline to file with the office of Attorney General Tom Reilly. Former Boston Mayor Raymond L. Flynn’s name tops the list of 30 people who signed the petition.

The initiative would define marriage as between a man and a woman. It is part of the effort to overturn same-sex marriage, which the Supreme Judicial Court legalized in 2003. Massachusetts is the only state to allow gay and lesbian marriages.

After the 2003 ruling, lawmakers hammered out a compromise ban, which would allow Vermont-style civil unions. That version passed in a joint session of the Legislature, called the Constitutional Convention, but must still be approved a second time.

In June, the Massachusetts Family Institute and a coalition of other groups opposed to same-sex marriage announced that they were launching the new effort to amend the state Constitution.

Kris Mineau, president of the group, said he was confident that the effort could gather the needed signatures, and would ultimately be approved by voters.

“We’ve got a broad grass-roots base. We had that base mobilized a year ago, during the Constitutional Convention, and that’s the same base we’re activating now to gear up for the signature drive,” he said.

Supporters of the new amendment drive pledged in June to actively seek the defeat of the earlier version. Mineau did so again last week, saying the pending amendment is “degrading to the institution of marriage.”

“We have to defeat the current amendment in order for our amendment to proceed,” he said.

Arline Isaacson, co-leader of the Massachusetts Gay and Lesbian Political Caucus, said supporters of the proposed amendment “are showing themselves to be extremists.”

“This measure would ban our marriages, deny us even civil unions. It’s mean-spirited. There are so many other issues they should be working on, like helping the poor and the needy,” he said.

Gov. Mitt Romney withdrew his support for the compromise ban, and is backing the new proposal. He said the compromise “muddied” the issue of same-sex marriage by legalizing civil unions.

After the new initiative is approved by Reilly’s office, proponents must gather about 66,000 certified signatures. Then the measure must be approved in two Constitutional Conventions in a row, before it would be put to the voters in 2008.