The Supreme Court agreed Friday to take up the explosive issue of same-sex marriage, thrusting itself into a policy debate that has divided federal and state governments and courts, as well as voters in nearly 40 states.
The high court's long-awaited decisions to hear challenges to the federal Defense of Marriage Act and California's Proposition 8 ban on same-sex marriage moves the issue to the top of the national agenda following a year in which advocates scored major legal and political victories.
The court likely will hear the cases in March and rule by late June on a series of questions, potentially including one of the most basic: Can states ban gay marriage? It also will decide whether gay and lesbian married couples can be denied federal benefits received by opposite-sex spouses.
Any decision will make history on an issue that has divided the nation for decades. Nine states and the District of Columbia now permit same-sex marriages, and a decision against California's ban would add the 10th and largest state. A ruling against the 1996 federal law could lead to a spike in gay marriages in all those states. Several more states are likely to consider allowing same-sex marriages in 2013.
By adding gay marriage to its docket, the Supreme Court assured that its 2012-13 term will be as historic as its last one, when it ruled on immigration and upheld President Obama's landmark health care law.
While a ruling in the California case could be confined to that state, it holds the most potential for a sweeping national precedent. If the state's ban is overthrown on constitutional grounds, it could negate some 31 other state bans on gay marriage as well.
Groups on both sides of the issue hailed the court's decision to wade into the same-sex marriage debate.
"It's been our belief all along that the ultimate fate of Proposition 8 will be in the hands of the Supreme Court," said Andrew Pugno, general counsel for the advocacy group ProtectMarriage, which sought the high court's intervention.
"I fully believe that this court's going to come down on the side of freedom and equality," said Chad Griffin, president of the Human Rights Campaign, which fights for gay, lesbian, bisexual and transgender rights.
The state Supreme Court cleared the way for gay marriages in California four years ago, and quickly some 18,000 couples took advantage. The voter initiative that passed several months later, however, has blocked more nuptials pending the completion of court action.
The original lawsuit against Proposition 8, filed by a gay couple and a lesbian couple who seek to marry, has racked up two lower court victories. The district court said the law violated the Constitution's equal protection clause because it was "premised on the belief that same-sex couples simply are not as good as opposite-sex couples." The appeals court ruled more narrowly that voters could not take away a right that gays and lesbians previously enjoyed.
In appealing to the Supreme Court, lawyers for ProtectMarriage went so far as to quote President Obama. Even as he endorsed gay marriage earlier this year, the president noted that millions of Americans "feel very strongly" about preserving the traditional definition of marriage because they "care about families."
Even if the court sides with gays and lesbians, opponents of same-sex marriage say it won't end the debate.
"The majority of Americans who have voted to protect marriage as the unity of a man and a woman are never going to go away," said Brian Brown, president of the National Organization for Marriage. A Supreme Court ruling in favor of same-sex marriage, he said, "would launch a national culture war."
In the Defense of Marriage Act challenge, the justices chose from among several overlapping cases, including challenges to the law from New England, New York and California. The plaintiffs in those cases - married couples, widows and widowers in states where gay marriage is or previously was legal - are seeking the same federal benefits accorded opposite-sex couples, from joint tax returns to survivor benefits.
The Defense of Marriage Act (DOMA) passed Congress after the so-called Republican Revolution of 1994 prompted President Bill Clinton to compromise on the issue. His signature didn't have much practical impact until Massachusetts started the same-sex marriage trend in 2004. It was followed by Maine, Vermont, New Hampshire, Connecticut, Iowa, the District of Columbia and, however briefly, California.
The law has two main sections, only one of which - defining marriage in federal laws as between men and women - is being contested. Because of it, benefits and programs enjoyed by opposite-sex couples aren't available to gays and lesbians under federal employment, health, tax and other laws. (The other provision shields states from having to recognize gay marriages from other states.)
Attorney General Eric Holder announced last year that the Obama administration considered DOMA unconstitutional and no longer would defend it in court. Into the breach stepped House Republicans, who argued - to no avail thus far - that the law saves needed federal resources and ensures they are distributed equally among the states.
Other arguments go closer to the difference between gay and straight couples: procreation. A coalition of 15 states filed a brief in one of the New England cases asserting that "the different procreative capacities of same-sex and opposite-sex couples support a constitutionally legitimate distinction for defining marriage and affording special benefits to its participants."
The central issue for advocates of same-sex marriage comes down to questions of due process and equal protection. They say prohibitions serve no governmental objective or legitimate purpose - they don't make life or marriage any better for heterosexuals.
In 10 consecutive lower court cases, federal judges have ruled on the side of gays and lesbians. In Nevada, a district court judge held last month that there is no constitutional right to same-sex marriage.
"That gives us enormous hope that the Supreme Court will swiftly affirm what these courts have all said," said Evan Wolfson, executive director of the advocacy group Freedom to Marry.
Advocates for gay rights applauded the court's agreement to hear a case and said a decision affirming or expanding on the lower court rulings would make history.
"I fully believe that this court's going to come down on the side of freedom and equality," said Chad Griffin, president of the Human Rights Campaign.
Those opposing same-sex marriage argue that it has no home inside the Constitution. They say gay couples can't procreate, so they can be treated differently. They say states and the federal government can't be forced to extend costly benefits to gays and lesbians.
Brian Brown, president of the National Organization for Marriage, said a decision in favor of same-sex marriage would set off "a constitutional revolution."
"The states don't have the right to obligate the federal government," he said. "They're forcing the federal government to obey state law."
The geographic deck remains stacked against gay marriage, but the tide may be turning. Voters in 31 states have banned same-sex marriages, and seven more states have laws barring those unions.
Still, 2012 has been a banner year for the gay rights movement. President Obama became the first president to endorse same-sex marriage. Voters in Maine, Maryland and Washington legalized it; Minnesota voters rejected a ban on it.
The past year has been "an extraordinary year of triumph and progress," Wolfson said. "Gays have not used up the marriage licenses. There's still plenty of marriage left, and no one has been hurt."
Several other states, from New Jersey to Hawaii, could join the gay marriage movement as early as next year. But the progress hasn't been without interruption. North Carolina voters banned gay marriage in May with 61% of the vote.