Boston ? Gov. Mitt Romney said Thursday he will support a proposed constitutional amendment banning gay marriage in Massachusetts, the only state where it is legal.

The Legislature was already working on a proposed amendment that would ban gay marriage but also would allow Vermont-style civil unions. The new proposal drops the civil union language, meaning such unions would remain illegal in the state.

If the new proposal passes procedural hurdles, it could appear on the statewide ballot as soon as November 2008.

Romney said the original proposed amendment – which was narrowly passed last year by state lawmakers and is awaiting a second round of voting later this year – is “muddied” because it includes both the gay marriage and civil union issues. He said voters should be able to decide on gay marriage with a “clean, straightforward, unambiguous amendment” that does not include civil unions.

“I believe it’s superior to the amendment which is currently pending before the state Legislature, and hope that this amendment will ultimately be the one which the citizens have the opportunity to vote upon,” Romney said.

The announcement was immediately decried by supporters of gay marriage.

Kris Mineau, right, president of Massachusetts Family Institute, speaks with Richard Regan, left, and Mary O'Donnell of Arlington, Mass., at the Massachusetts Statehouse in Boston on Thursday after a news conference. The Coalition for Marriage and Family discussed a citizens initiative for a constitutional amendment banning both same-sex marriages and civil unions in Massachusetts, the only state where gay marriage is now legal.

“I think the amendment is an extreme and mean-spirited effort to take away marriage equality and replace it with nothing,” said Mark Solomon, political director of MassEquality. “It is really sad that (Romney) is playing politics with gay and lesbian families and their children, and that’s all that is.”

The new proposed amendment will take the form of a citizen’s initiative. That means the state attorney general’s office must sign off on the proposal language, supporters must collect 65,825 voters’ signatures and one-quarter of lawmakers in the Legislature must vote twice to approve it. Only if it passes those hurdles would the state’s voters have their say.

Boston Archbishop Sean O’Malley and the bishops of the state’s other three Roman Catholic dioceses pledged support for signature-gathering efforts.

“We encourage all Catholics to exercise their civil right to participate in the signature drive for the new initiative petition,” they said in a statement.

Massachusetts’ highest court ruled in November 2003 that the state constitution guaranteed same-sex couples the right to marry. The nation’s first state-sanctioned, same-sex weddings began taking place May 17, 2004, and since then, thousands of couples have tied the knot.

The two proposals now on the table are both efforts to reverse the court’s gay marriage ruling via a constitutional amendment.