In a presidential election season, politicians will find their stance on same-sex couples will be critical.

We have made a powerful statement, Democrat Governor Andrew Cuomo said last week after four Republican senators broke ranks, crossed the floor in the New York Assembly and gave Democrats a slim 33-29 majority in a bill, approved by the lower house, to legalise gay marriage.

With gay marriage emerging as a red hot national issue, it remains to be seen just how powerful New York's law will prove.

After days of cliff-hanging suspense and protests, the bill became law on June 24, doubling the number of Americans, to 11 per cent of the population, who live in states - the others are Iowa, New Hampshire, Massachusetts, Vermont and Connecticut, plus the District of Columbia - where gay marriage is legal. It goes into effect on July 25, giving same-sex couples rights already enjoyed by heterosexuals.

"There really aren't legitimate legal reasons for the state to deny me and my partner the rights and responsibilities associated with marriage that are given to thousands of New Yorkers every year," Daniel O'Donnell, the openly gay Democrat who sponsored the Marriage Equality Act, said.

As revellers across the nation and at New York's annual Gay Pride parade celebrated, advocates - intent on reform via the judiciary, the state's and at federal level - wondered if New York would be a game-changer in a fight between gays and conservatives for the hearts and minds of Americans.

Crucially, New York is the first state where a Republican-controlled legislature has passed a same-sex marriage law.

"It gives the national conversation a new start," says Julie Bolcer, New York correspondent for gay magazine Advocate. "It may impact nearby states, like Maryland and Rhode Island, that have been on the cusp of having marriage equality, or were close. And that could follow the New York legislative model to success in the next year or so."

Gay marriage, and the belief Americans should enjoy equal liberties, the impulse that drives all civil rights campaigns, is moving to centre stage in the United States. The issue will be harder for politicians to duck in a presidential election season, where advocates sense a breakout into the US heartland and opponents want to push back gay victories and prevent same-sex unions via a constitutional amendment.

The first test may be in Maryland where advocates want to restart a same-sex marriage bill that faltered in February, partly because Democrat Governor Martin O'Malley did not provide the public leadership shown by Cuomo in New York.

The bill passed the Senate but failed by a handful of votes in the House, in part, due to opposition from black churches and conservative Democrats and Republicans. O'Malley, a Catholic, sought a compromise in civil unions.

Bolcer says the New York model - strong leadership, powerful backing and an organised legislative strategy - may work in Maryland and Rhode Island where polls indicate a "slight majority in favour of marriage equality".

That edge will intensify efforts by both sides, as New York's law propels same-sex marriage into national contention during the 2012 presidential election race.

"The question is on the table," says Bolcer. "And everyone in the race will have to sharpen his or her opinion on it."

However, New York's so-called Rockefeller Republicans represent the GOP's more moderate voice, unlike Tea Party conservatives who garner national attention on Fox News.

"I don't think you can draw a straight line between the Republican Party in New York and Michelle Bachmann," says Bolcer. Bachmann, who this week entered the race to be the Republican candidate in next year's presidential elections, wants a constitutional amendment banning same-sex marriage.

At the same time the National Organisation of Marriage (NOM) says it will spend US$2 million ($2.4 million) to reverse the New York vote, targeting turncoat Republicans who voted with Democrats. A poll by the organisation says 57 per cent of voters oppose the law.

"We're going to make sure the people are made accountable," vowed NOM president Brian Brown, a slap at GOP senators Stephen Saland, Mark Grisanti, James Alesi and Roy McDonald, whose support guaranteed the bill's passage.

Gay campaigners hope support from upstate, suburban GOP senators and funding from Wall Street Republicans mean they have tapped into a new political matrix.

"You get to the point, where you evolve in your life, where everything isn't black and white, good and bad, and you try to do the right thing," McDonald told the Los Angeles Times. The dissidents were given cover by a clause exempting religious groups from performing same-sex marriages.

Yet while the law was a victory it has an Achilles heel, an "inseverability clause" that means no part can be refuted without the entire law apparently becoming null and void, a worry for gay advocates as opponents prepare their appeals.

For gay marriage supporters have a major fight on their hands. On a federal level the Defence of Marriage Act bans same-sex marriage, denying benefits to gay couples. The law also allows any state to ignore another state's unions. Forty-one states have constitutional bans or laws prohibiting same-sex marriage. In 29 states workers can be fired for being gay, while in 36 states they can be fired for their gender identity. Voter initiatives have also opposed gay marriage, and in 2008 California's Proposition 8 said that only a man and woman could be legally married.

Prop 8 vanquished a gay marriage law passed six months earlier, throwing 18,000 Californian gay marriages into legal jeopardy, a reversal being appealed in the courts.

The New York law was opposed by Catholic bishops, who feared for marriage and family, while the NOM warned of a "profound and irreconcilable conflict". "I think it will energerise the conservative base enormously," the NOM chairwoman Maggie Gallagher said. She said it "wasn't over in New York", and predicted battles in other states as the issue was injected into the presidential campaign.

Nonetheless, same-sex marriage campaigners sense the tide is finally flowing their way, with a slow shift in public attitudes towards America's LBGT [Lesbian, Bisexual, Gay, Transgender] community. In a national Gallup poll in May a majority of Americans, 53 per cent, supported same-sex marriage for the first time. This transformation is partly demographic: gay marriage draws most support from younger people.

While only six states allow same-sex marriage, eight - California, New Jersey, Nevada, Washington, Oregon, Illinois, Hawaii and Delaware - allow civil unions. Today gay marriage is legal for 35 per cent of Americans, up from zero in 2004 when Massachusetts first legalised same-sex marriage.

Nationally, gays take heart in support from President Obama, who ended the Pentagon's "Don't Ask, Don't Tell" policy on homosexuals serving in the military, signed a hate crime law that makes it a federal crime to attack someone because of their sexual preference, and advocates full civil rights for the LBGT constituency.

But the President has stopped short of support for same-sex marriage. He says his policy is "evolving". Previously, Obama said he opposed same-sex unions on religious grounds. But his views, like those of Americans, are shifting. Obama says he is a "fierce advocate" for gays and opposes Prop 8. Gay advocacy groups have kept up pressure with a "Say I Do" petition and an "EvolveAlready" Twitter campaign.

"He didn't use the word marriage," says Bolcer, who covered Obama's appearance at a LBGT fundraiser. "But he has moved away from using the phrase civil unions." Such nuances, she suggests, are necessary to pacify Obama's base. He may appeal to the young, but many African-American and Latino Democrats hold conservative views on marriage.

While Obama left the issue of gay marriage vague, he said his Administration would not defend the constitutionality of DOMA, leaving it to states to decide "how best to uphold the rights of their own citizens".

Republican presidential hopefuls have signalled a very different stance. Bachmann, the Tea Party darling, told Fox News she would support a constitutional amendment defining marriage as "between a man and a woman".

During a CNN debate for GOP presidential aspirants in New Hampshire [which holds the first Republican primary election in the 2012 race and garners major media coverage], her rivals - Tim Pawlenty, Ron Paul, Herman Cain, Newt Gingrich, Mitt Romney and Rick Santorum - prevaricated furiously as they tried to triangulate state and individual rights with religious conservatism. Five supported Bachmann's stance on a constitutional amendment.

But primaries often attrAct Party extremists. "These candidates are looking to make it past the primary, and that's why they are messaging to extreme right voters," says Fred Sainz, who represents Human Rights Campaign. Talk of a constitutional amendment is "pure demagogy" he says. "Plenty of conservative Republicans disagree with the enshrinement of discrimination in the constitution."

Meanwhile, the constitution is cited in two major cases challenging gay marriage bans. Perry v Brown is aimed at Proposition 8. Gill v Office of Personnel Management targets DOMA. Both seem destined for the US Supreme Court.

What is missing is the broader debate presaged in 2003 by English gay writer and political conservative Andrew Sullivan. "Conservatives have long rightly argued for the vital importance of the institution of marriage for fostering responsibility, commitment and the domestication of unruly men," he wrote in Time. "Bringing gay men and women into this institution will surely change the gay subculture in subtle but profoundly conservative ways."

It is this reasoned territory gay marriage advocates must occupy to dispel the dark fears harnessed by opponents. "When the only result [of gay unions] is tomorrow is more just and equal than yesterday," asks Sainz, "then why shouldn't we support it? It's part of the natural evolution of American life. When we know better, we do better."

The stats
53% of Americans support same-sex marriage, according to a Gallup poll in May

6 states allow same-sex marriage

8 allow civil unions

41 states have constitutional bans or laws prohibiting same-sex marriage

29 states where workers can be fired for being gay