The California Supreme Court voted unanimously on Wednesday to decide the question of whether supporters of Proposition 8, the ballot measure that repealed California’s same-sex marriage law, have legal standing to appeal a federal court decision last year invalidating the ballot measure.

In a statement released late Wednesday, a court spokesperson said the state Supreme Court would expedite the process for receiving legal briefs and planned to schedule oral arguments for the case in September.

Its decision, expected later in the year, would determine whether same-sex marriage opponents can proceed with their appeal of a U.S. District Court decision last year invalidating Proposition 8 on grounds that it violates the U.S. Constitution by denying same-sex couples the right to marry.

The legal standing issue surfaced in 2009 when former California Gov. Arnold Schwarzenegger and then-Attorney General Jerry Brown, now the governor, refused to defend Prop 8 against a lawsuit filed by two same-sex couples.

The private citizens who organized Prop 8 were allowed to intervene on its behalf during the district court proceedings, including a civil trial. But gay rights attorneys challenging the ballot measure on appeal argued that only the state could defend the measure because it was a state law.

On Jan. 4, the Ninth Circuit U.S. Court of Appeals sidestepped a ruling on whether the district court was correct in declaring Prop 8 unconstitutional and sent the lawsuit filed by the gay couples to the state’s Supreme Court for an advisory ruling on the standing question.

If the California Supreme Court rules Prop 8 supporters lack legal standing, the district court ruling would take effect, allowing same-sex couples to marry as they had during the short period of time before voters approved Prop 8 in the November 2008 election.

However, most legal observers believe the Prop 8 backers would respond immediately by asking the U.S. Supreme Court to issue a stay to prevent same-sex marriages from resuming until the U.S. Supreme Court decides whether it would take the case on its merits.

Most legal observers believe the case will ultimately wind up in the U.S. Supreme Court.

Meanwhile, if the California Supreme Court rules in favor of granting legal standing to Prop 8 backers, the case would go back to the Ninth Circuit appeals court, which would then hear the case on the merits of whether Prop 8 violates the U.S. Constitution, as gay rights attorneys and the same-sex couples who brought the lawsuit claim it does.

In its statement released today, the California Supreme Court gave a legal explanation of the question it says it will answer after deliberating over the case:

“Whether under Article II, Section 8 of the California Constitution, or otherwise under California law, the official proponents of an initiative measure possess either a particularized interest in the initiative’s validity or the authority to assert the state’s interest in the initiative’s validity, which would enable them to defend the constitutionality of the initiative upon its adoption or appeal a judgment invalidating the initiative, when the public officials charged with that duty refuse to do so.”

The court’s statement says Prop 8 supporters’ attorneys must file their opening brief for the case by March 14, and attorneys representing the same-sex couples seeking to invalidate Prop 8 must file their response by April 4.

“The California Supreme Court shortened the normal briefing schedule to expedite consideration and resolution of the issues in the matter and to accommodate oral argument as early as September 2011,” the statement says.

Jennifer Pizer, an attorney for Lambda Legal, an LGBT legal advocacy group that’s monitoring the Prop 8 case, called on the California Supreme Court to confirm that Prop 8 backers don’t have legal standing to appeal the case.

Prop 8 supporters “are not law enforcers and have the same limited rights as everyone else to litigate only when their own rights are at stake, not merely to assert their opinions about others’ rights,” Pizer said in a statement.

“Initiative proponents also cannot step into the shoes of the attorney general, the governor or other state officials,” she said. “The reason for this is basic: the governor and attorney general are elected by the people to represent all the people, not just one point of view on one issue, out of countless, competing concerns.”

A spokesperson for Prop 8 supporters, including the National Organization for Marriage, which campaigns against same-sex marriage rights, could not be immediately reached for comment.

Chad Griffin, president of the American Foundation for Equal Rights, which is financing the same-sex couples’ lawsuit seeking to overturn Prop 8, said his group is hopeful that they will prevail and same-sex marriage will be declared a constitutional right.

“AFER is challenging Prop 8 not only for our plaintiffs, two loving couples who want to marry, and not only for the thousands of loving couples like them, but for the simple reason that our laws should treat everyone equally,” he said.