(CNN) - It could be the social issue that defines President Barack Obama's second term legacy: same-sex marriage.
He faces a monumental choice on Thursday over whether to put the force of his office behind the idea that gays and lesbians have a constitutional right to marry.
Government sources say the Justice Department will by day's end articulate a legal position in the so-called Proposition 8 case, a ban by California voters over same-sex marriage that is now being challenged in the Supreme Court.
That case and another appeal over the federal Defense of Marriage Act will produce blockbuster rulings from the justices in coming months.
Gay rights groups have privately urged Obama and his top aides to go beyond his previous personal rhetoric in support of the right and come down "on the side of history" in this legal fight.
Those sources tell CNN that Obama has made the final decision over whether to file a brief and what to say.
As of earlier this week, there was still internal debate among White House and Justice Department staff about whether the president should take the big step and say there is a constitutional right of gay and lesbian couples to wed.
The administration was also considering a compromise position -- affirming previous support for same-sex marriage, while conceding states may have the option to ban it.
The issue could be a defining moment in Obama's presidency, similar to the political impact last year when the Supreme Court upheld the health care reform law he spearheaded.
He must decide how much political capital to expend in coming months when expressing his views and those of the executive branch.
Obama has already faced strong opposition on the issue from many Republican state and congressional lawmakers, as well as social conservatives.
The justices will hear oral arguments in the Proposition 8 case March 26, with a ruling due by the last week of June.
The separate case over the Defense of Marriage Act involves a 1996 law that says for federal purposes, marriage is defined as only between one man and one woman.
That case will be argued March 27.
But it is the Proposition 8 case where the high court is being asked to establish the constitutional "equal protection" right.
Though technically not a party in the California appeal and not required to weigh in, government sources say the administration would file an amicus or "friend of the court" brief.
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