The US Supreme Court is to decide whether or not same-sex couples have a right to marry under the country's constitution, setting up a potentially historic ruling on one of America's most sweeping social issues.
The justices will take up gay rights cases that ask them to overturn bans in four states and declare for the entire nation that people can marry the partners of their choice, regardless of gender. The cases will be argued in April and a decision is expected by late June.
Supporters of same-sex marriage said they expected the court to settle the matter once and for all.
Attorney general Eric Holder said the Obama administration would urge the court “to make marriage equality a reality for all Americans”.
Advocates for traditional marriage want the court to let the political process play out, rather than have judges order states to allow same-sex couples to marry.
"The people of every state should remain free to affirm marriage as the union of a man and a woman in their laws," said Austin Nimocks, senior counsel for anti-gay marriage group Alliance Defending Freedom.
Momentum has shifted dramatically in the United States in recent months in favour of gay marriage. Same-sex couples now can marry in 36 states and the District of Columbia.
That number is nearly double what it was just three months ago, when the justices initially declined to hear appeals from five states seeking to preserve their bans on same-sex marriage. The effect of the court’s action in October was to make final several pro-gay rights rulings in the lower courts.
Now the court will be weighing in on major gay rights issues for the fourth time in in 27 years as societal norms have been sharply redefined.
In the first of those, in 1986, the court upheld Georgia’s anti-sodomy law in a devastating defeat for gay rights advocates. But the three subsequent rulings, all written by Justice Anthony Kennedy, were major victories for gay men and lesbians.
In its most recent case in 2013, the court struck down part of a federal anti-gay marriage law in a decision that paved the way for a wave of lower court rulings across the country in favor of same-sex marriage rights.
For this latest case, the court is extending the time it usually allots for argument from an hour to two and a half hours. The justices will consider two related questions: whether the US Constitution requires states to issue marriage licences to same-sex couples and whether states must recognise same-sex marriages performed elsewhere.
The appeals before the court come from gay and lesbians in Kentucky, Michigan, Ohio and Tennessee.
The federal appeal court that oversees four states upheld their same-sex marriage bans in November, reversing pro-gay rights rulings of federal judges in all four states. It was the first, and so far only, appellate court to rule against same-sex marriage since the high court’s 2013 decision.
One of the plaintiffs from Ohio, James Obergefell, said he was crying "tears of joy and sadness" after the court accepted his appeal. In 2013 he flew to Maryland with his dying partner John Arthur, so they could marry before his death. The couple sued to force Ohio to list Mr Arthur as married on his death certificate, which would allow the men to be buried next to each other. Mr Arthur died 15 months ago.
“I can’t wait to walk up those steps and have the Supreme Court understand that we’re just like everyone else,” Mr Obergefell said.
Other plaintiffs are Gregory Bourke and Michael Deleon, who have been together for 32 years. They were married in Canada in 2004 and now live in Kentucky.
“Our family is like any other family. We have children, we have jobs, we have lives, we are very much engaged in our community and yet we don’t feel like we are being treated yet as equal citizens,” Mr Bourke said.
Ten other states also prohibit same-sex unions. In Arkansas, Mississippi, Missouri, South Dakota and Texas, judges have struck down anti-gay marriage laws, but they remain in effect pending appeals. In Missouri, same-sex couples can marry in St Louis and Kansas City only.
Louisiana is the only other state that has seen its gay marriage ban upheld by a federal judge. There have been no rulings on lawsuits in Alabama, Georgia, Nebraska and North Dakota.
PA