Gay marriage took another step Tuesday on its march to the U.S. Supreme Court, when a federal appeals court that struck down California's ban on same-sex unions refused to reconsider the ruling.

Now that the case has run its course in the 9th U.S. Circuit Court of Appeals, the measure's sponsors "absolutely" plan to take the case to the high court, said Brian Raum, a lawyer with the Alliance Defense Fund, a Christian legal defense group.

Backers of the ban, known as Proposition 8, now have 90 days to petition the Supreme Court to review the finding that the ban violates the civil rights of gay men and lesbians in California.

If at least four justices agree to accept the case, oral arguments would likely be held next spring.

The developments came after the 9th U.S. Circuit declined to review a February ruling by two of its member judges who found the state's voter-approved same-sex marriage ban was unconstitutional, in part because it rescinded a right that gay and lesbian Californians already had won.

Same sex unions briefly were legal in the state before 52 percent of voters approved the ban in November 2008.

Gay marriage supporters welcomed the latest news in the long-running legal battle. If the Supreme Court refuses to take up the case and lets the appellate ruling stand, same-sex marriages could be legal again in California by the end of the year.

"The final chapter of the Proposition 8 case has now begun," said American Foundation for Equal Rights co-founder Chad Griffin, whose group is funding the effort to overturn the ballot measure. "Should the United States Supreme Court decide to review the 9th Circuit's decision in our case, I am confident that the justices will stand on the side of fairness and equality."

A majority of the 9th Circuit's 26 actively serving judges voted against giving the case a second look while leaving Proposition 8 in effect until a Supreme Court appeal is resolved.

Judge Diarmuid O'Scannlain issued a terse dissent, arguing that the full 9th Circuit should have reexamined its panel's 2-1 decision because in his view it was based on a "gross misapplication" of Supreme Court precedent and "overruled the will of seven million California voters." Judges Carlos Bea and Jay Bybee joined him in that opinion.

The 9th Circuit does not often agree to rehear cases, a procedure known as en banc review. Federal court rules reserve the practice for appeals that involve "a question of exceptional importance" or if the original decision appears to conflict with Supreme Court or 9th Circuit precedents.

Several other high-profile same-sex cases also are moving toward the high court. A three-judge panel of the Boston-based 1st U.S. Circuit Court of Appeals declared last week that the federal law that prohibits recognition of same-sex couples unconstitutionally denies Social Security and other federal spousal benefits to married gay couples.

The Massachusetts and California cases could reach justices at the same time, which "probably increases the likelihood the court will take the (Proposition 8) case," said David Boies, a lawyer representing the two unmarried couples who first sued to overturn the ban three years ago.

At the same time, because the 9th Circuit limited its decision to California instead of ruling that gay marriage bans are inherently unconstitutional, the Supreme Court might be inclined to let it stand, he said.