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The same-sex marriage movement is about to face a critical test, as New Hampshire lawmakers prepare to vote on a proposal to repeal the state's 2009 gay marriage law.

With a vote expected on the House floor as early as Wednesday, foes and supporters of the law are clashing in a battle over whether New Hampshire will be the first state to reverse the tide of same-sex marriage with a legislative vote. The debate marks a sharp contrast to the landmark decision last summer to legalize gay marriage in nearby New York, the largest state to approve the unions.

"It's very significant," New Hampshire state Rep. David Bates, the bill's sponsor, told FoxNews.com. "This will be the first place ever, anywhere in the world, where a legislature has reversed its position on same-sex marriage. ... That hasn't happened anywhere."

Yet the bill's chances are unclear. Republicans have an overwhelming majority in both chambers of the New Hampshire legislature, but not all Republicans are on board with Bates' bill. And Democratic Gov. John Lynch has vowed to veto, in which case repeal supporters would need to rally a two-thirds majority to override the governor's objections.

Bates said he expects the bill to pass the legislature, but that the "real question" is over the veto override.

The debate has attracted the interest and involvement of state and national organizations on both sides. The vote could have broader implications. Though some hailed the New York vote last summer as a sign that other states would soon follow suit, a successful repeal in New Hampshire would mark a setback for the movement -- and hardly the first. Maine voters in 2009 voted to stop their state's same-sex marriage law from going into effect. In California, voters likewise rejected same-sex marriage after a court ruled in favor of it in 2008. The case is still tied up in court.

The movement has struggled to break out of the northeast. Aside from New Hampshire and New York, other states that allow gay marriage are Massachusetts, Connecticut and Vermont, as well as Iowa and the District of Columbia. Yet in Iowa, voters in 2010 ousted the judges responsible for that decision.

The Human Rights Campaign, a gay-rights advocacy group, is urging supporters to fight the New Hampshire legislation. A new appeal from the group likened it to "legislating bigotry."

"We need to show New Hampshire that the rest of the country is watching," the group said.

A state group, Standing Up for New Hampshire Families, has cited recent polls showing a majority of those surveyed oppose stripping the marriage law.

"How much more evidence do our legislators need to realize that repealing marriage equality -- which has allowed more than 1,800 Granite State couples to marry -- is a bad idea and one that voters overwhelmingly reject?" Craig Stowell, co-chairman of Standing Up for New Hampshire Families, said in a statement.

Organization spokesman Tyler Deaton said the 2009 law "treats all New Hampshire families equally," and that most voters oppose "any effort to repeal or dilute it."

But Bates said the "visceral" reactions in Maine, California and Iowa show the people are not in favor of gay marriage.

"If this were to be successful here, it just adds to that clear indication of what the citizens across the country want," he said.

As for those who are already married, Bates said the New Hampshire bill would have "absolutely" no effect on their marriages. "This will not be nullifying any of the marriages that have taken place in the last couple of years," he said.

But going forward, the proposal would prohibit same-sex marriages and instead permit civil unions for gay couples -- which is what the state allowed before the 2009 law.

A House committee last fall approved the repeal proposal, recommending it to the full House. The panel also recommended against another bill that would repeal gay marriage without a civil union replacement.

The New Hampshire House in early 2010 rejected attempts to repeal or ban same-sex marriage in the state, but that was before Republicans made big gains in the November elections.