When a coalition of gay rights advocates challenged California’s voter-approved ban on same-sex marriage three years ago, they made no secret of their ultimate goal: force the U.S. Supreme Court to tackle the same-sex marriage issue.

Now, the legal showdown over Proposition 8 is headed to the nation’s highest court. And most legal experts predict the justices will take up the case and for the first time decide the legality of a state law forbidding same-sex nuptials.

In a brief order Tuesday, the 9th U.S. Circuit Court of Appeals refused to reconsider its ruling this year that struck down Proposition 8, finding the 2008 law unconstitutional because it stripped gay and lesbian couples of the right to marry.

The appeals court denied a request from Proposition 8 backers for a rehearing before a special 11-judge panel. Under the court’s order, same-sex couples will not be allowed to marry during the 90 days Proposition 8 supporters have to appeal to the Supreme Court.

That next step is inevitable. Andy Pugno, general counsel for ProtectMarriage.com, the measure’s sponsor, vowed to “promptly” file the petition, saying supporters look forward to a “positive outcome” in the Supreme Court.

As a result, the Supreme Court would jump headlong into the national debate over gay marriage in its next term, which will begin on the eve of the November presidential election. The 9th Circuit’s order comes less than a week after a federal appeals court in Boston found the federal government’s ban on same-sex marriage rights unconstitutional, heightening the prospect the Supreme Court may be forced to consider both state and federal gay marriage restrictions.

If the justices take up the California legal battle, they would be expected to hear arguments early next year and rule by June 2013.

Kristin Perry and Sandy Stier, of Berkeley, one of two couples challenging Proposition 8, said Tuesday they are ready for their fight to reach the high court. They had hoped to have their case resolved before their twin sons graduate from high school, but it appears the outcome may coincide with their graduation next year.

“For me, it feels like it’s the decision that launches the case out of California to the national stage,” Perry told this newspaper.

The 9th Circuit’s action was widely expected. A majority of the court’s 25 full-time judges must vote to rehear the case with an 11-judge panel, but the court is dominated by Democratic appointees who were considered less inclined to take a fresh look at the case.

Three conservative judges — Diarmuid O’Scannlain, Carlos Bea and Jay Bybee — dissented, saying they would reconsider February’s three-judge ruling. Judge N. Randy Smith, who dissented from that ruling, also would have reheard the case but did not sign O’Scannlain’s dissent.

In its earlier 2-1 decision, the 9th Circuit invalidated Proposition 8, saying it took away the legal right to marry without any social or legal justification other than bias against gays and lesbians. The decision upheld former Chief Judge Vaughn Walker, who presided over an unprecedented trial in early 2010 and declared the law unconstitutional.

Legal experts, however, have noted the narrowly crafted decision avoided finding a constitutional right for same-sex couples to wed. Instead, the 9th Circuit ruling could be limited to California’s unique circumstances — voters taking away the right to marry established in spring 2008, when the state Supreme Court struck down the state’s previous same-sex marriage ban.

That decision allowed more than 18,000 gay and lesbian couples to marry before voters approved Proposition 8, and those marriages remain intact. Ninth Circuit Judge Stephen Reinhardt stressed that point to say the state cannot create separate, unequal classes of relationships without violating the constitution.

Reinhardt and Judge Michael Daly Hawkins, who joined in the decision, said in Tuesday’s brief order that their ruling was confined to California.

“We did not resolve the fundamental question that both sides asked us to: whether the Constitution prohibits the states from banning same-sex marriage,” the two said. “That question may be decided in the near future, but if so, it should be in some other case, at some other time.”

While the 9th Circuit did not specifically establish a broader fundamental right for same-sex couples to marry, legal experts say the Proposition 8 case could still pose that question to the Supreme Court. The 9th Circuit relied heavily on a 1996 Supreme Court ruling, written by Justice Anthony Kennedy, striking down a Colorado law that denied gays and lesbians protection against discrimination.

Kennedy is considered a crucial swing vote on the gay marriage question.

Former U.S. Solicitor General Theodore Olson, a lawyer for the couples, said he would argue in the “broadest possible way” when the case reaches the Supreme Court that same-sex couples should have the equal right to marry.

Carl Tobias, a University of Richmond law professor, said there should be the four votes necessary for the Supreme Court to take the case. Marc Spindelman, an Ohio State University law professor, agreed.

“I do think there is pressure on the more conservative justices on the Supreme Court to get this issue heard and decided sooner rather than later,” he said.

The Proposition 8 case is on the same track as the federal case decided last week in Boston, which struck down the 1996 Defense of Marriage Act. The 9th Circuit is expected to hear a similar DOMA case in the fall.

The Supreme Court could avoid the larger constitutional questions in the Proposition 8 case. There is lingering doubt over whether Proposition 8 supporters have a legal right to appeal when the state’s top elected leaders, Gov. Jerry Brown and Attorney General Kamala Harris, refuse to do so. The 9th Circuit and state Supreme Court allowed the measure’s proponents to defend the law, but the U.S. Supreme Court could take a different view.

The Obama administration has not taken a position in the Proposition 8 case, but it has refused to defend the federal ban and deemed it unconstitutional. President Barack Obama recently announced his support for marriage rights for gay and lesbian couples.

Howard Mintz covers legal affairs. Contact him at 408-286-0236. Follow him at Twitter.com/hmintz.