BOSTON (AP) – The debate over same-sex marriage in Massachusetts has taken a noisy turn after a coalition of Catholics and conservative organizations, with the blessing of Gov. Mitt Romney, vowed to block future same-sex unions with a new and uncompromising grass-roots appeal to voters.

The new campaign, backed by deep-pocketed national organizations, seeks to scuttle an existing ballot initiative that lawmakers passed last year that would ban same-sex marriage while legalizing Vermont-style civil unions.

Instead, the coalition will try to undo lawmakers’ work last year by urging them to vote against the proposed ballot question this fall, while gathering petitions to replace it with another, streamlined question that makes no provisions for civil unions.

While prospects of the new effort are unclear, supporters and opponents agree on one thing: The new effort represents a change in tactics that is sure to alter the political landscape for same-sex marriage in the nation’s only state that allows it.

Evelyn Reilly, director of public policy at the Massachusetts Family Institute, one of the groups supporting the new initiative, said it should come as no surprise that same-sex marriage opponents are taking a new tack.

“I think it’s very significant, because if the other amendment goes through, we’ve really accomplished very little,” she said. “You save the name of marriage, but that’s it.”

She said she doubted that the coalition, which includes both local organizations like the Black Ministerial Alliance as well as high-profile national groups like Focus on the Family and the Family Research Council, will have trouble gathering the roughly 66,000 voter signatures needed to put the new referendum on the ballot.

“We had so many people activated during the last marriage drive. We had thousands of people coming to the legislature,” she said. “I’ve never seen so much activity. The legislators had never [seen] so much activity.

“Not that it’s going to be easy. It’s a huge undertaking,” she added.

The amendment, which won initial approval last year, needs to win the backing of at least half of all 200 lawmakers again this session before landing a spot on the 2006 statewide ballot. The new initiative also needs to be approved twice, once during the current session and once during the next. The soonest it could get on the ballot is 2008.

Because the second amendment is starting life as a citizen’s initiative, it will only need to win the backing of one-fourth of lawmakers – provided backers can first obtain the voter signatures.

Rep. Emile J. Goguen, D-Fitchburg, a same-sex marriage opponent, said the new initiative has complicated what would have otherwise been a fairly swift resolution to same-sex marriage. Waiting until 2008 will muddy the matter, as more gay and lesbian couples marry, he said.

“I think it’s a mistake if they do it that. Go along with the original amendment for 2006,” he said. “They’re just prolonging the will of the people … The general consensus of opinion is do it, and do it now.”

Josh Friedas, advocacy director for the Massachusetts Freedom to Marry Coalition, said the new initiative has raised the stakes, as national conservative groups enter the fray.

“This means that the civil rights community in Massachusetts is going to have to raise substantially more money than it ever has raised before, and we’re really going to have to be fighting two campaigns simultaneously,” he said.