Will Obama Join Legal Fight for Gay Marriage?
PHOTO: An opponent of Prop 8 holds an American flag and gay pride flag during a demonstration outside of the Ninth Circuit Court of Appeals in this Dec. 6, 2010 file photo in San Francisco, Cali.

There is one mystery left to solve before the Supreme Court gathers to hear a challenge to Proposition 8 in late March.

That is: Will the Department of Justice weigh in on the case in favor of opponents of Prop 8, the California ballot measure that defined marriage as between one man and one woman? And what will it say?

Hollingsworth v. Perry concerns the California ballot initiative, enacted in 2008. In a brief filed with the court on Thursday, opponents of Prop 8 made broad arguments claiming that it is unconstitutional.

"Proposition 8 is an arbitrary, irrational and discriminatory measure that denies gay men and lesbians their fundamental right to marry in violation of the due process and equal protection clauses," the opponents said.

Related: Court Prepares for Affirmative Action, DNA Cases

The brief was written by Theodore B. Olson and David Boies, who represent the American Foundation for Equal Rights.

They wrote, "Because of their sexual orientation -- a characteristic with which they were born and which they cannot change -- plaintiffs and hundreds of thousands of gay men and lesbians in California and across the country are being excluded from one of life's most precious relationships.

"They may not marry the person they love, the person with whom they wish to partner in building a family and with whom they wish to share their future and their most intimate and private dreams," they added.

The arguments tackled more than just the ruling that struck down Prop 8 on narrow grounds specific to California's history on the issue of gay marriage.

The language was sweeping: "The only substantive question in this case is whether the state is entitled to exclude gay men and lesbians from the institution of marriage and deprive their relationships -- their love -- of the respect, and dignity and social acceptance, that heterosexual marriages enjoy.

"This badge of inferiority, separateness and inequality must be extinguished. When it is, America will be closer to fulfilling the aspirations of its citizens," they wrote.

A month ago, supporters of Prop 8 filed their brief in the case. Read it HERE.

While the administration will certainly weigh in on the other gay marriage case in front of the court -- a challenge to the federal law, the Defense of Marriage Act (DOMA) -- it has never filed a brief in the Prop 8 case because it was not directly involved.

While the DOMA case challenges a federal law that denies federal benefits to same-sex couples who are legally married in their state, the Prop 8 case asks a much broader question: Is there a fundamental right to gay marriage under the Constitution.

Solicitor General Donald B. Verrilli Jr. is not required to file a so-called "friend of the court" brief in the Prop 8 case, but sources said the administration is considering the possibility at the highest levels. If it chooses to weigh in, it has to do so by the last week of February.

In San Francisco Wednesday, President Obama told ABC News station KGO-TV, "The solicitor general is still looking at this. I have to make sure I'm not interjecting myself too much into this process, particularly when we're not party to the case. I can tell you, though, that obviously my personal view is that I think that same-sex couples should have the same rights and be treated like everybody else."

Related: Read More About Prop. 8

At the White House briefing on Thursday, White House spokesman Jay Carney said, "I think you have seen no expression from the president on the constitutional or legal aspects of this. He has an opinion, obviously, about Proposition 8 as policy, but we have no comment and nothing to say at this point about an issue that is properly looked at as a legal and constitutional matter over at the Department of Justice."

Back in May, the president told ABC News' Robin Roberts that he had evolved on the issue of gay marriage and had come to personally support it. He seemed to suggest in the interview, however, that he felt the issue should be left up to the states.

"I continue to believe that this is an issue that is going to be worked out at the local level because, historically, this has not been a federal issue, what's recognized as marriage," the president said.

But at the inauguration, he used much broader language:"Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law -- for if we are truly created equal, then surely the love we commit to one another must be equal, as well."

Even if the Department of Justice files a brief, it doesn't have to get to the broad question. It could, for example, limit its arguments to whether the supporters of Prop 8 -- in the absence of California officials who have refused to defend it -- have a legal right to bring the case. Or, it could argue in favor of the relatively narrow opinion from the 9th Circuit Court of Appeals.

In a conference call on Thursday, Olson urged the administration to step in.

"We very much would like the United States, in the form of the solicitor general, to file a brief," he said.

Olson, who served as the solicitor general under the George W. Bush administration, added that the justices pay particular attention when briefs come in on behalf of the government.

More ABC News