By Peter Henderson and Dan Levine
SAN FRANCISCO (Reuters) - A U.S. appeals court is set to rule on Tuesday whether California's ban on gay marriage is constitutional in a case that is likely to lead to a showdown on the issue in the U.S. Supreme Court.
California joined the vast majority of U.S. states in outlawing same-sex marriage in 2008, when voters passed the ban known as Proposition 8.
The socially conservative vote by a state more known for hippies and Hollywood was seen as a watershed by both sides of the so-called culture wars, and two gay couples responded by filing the legal challenge currently making its way through the federal courts.
A federal judge in San Francisco struck down Proposition 8 in 2010, and gay marriage opponents appealed that ruling to the 9th U.S. Circuit Court of Appeals. Regardless of how the 9th Circuit decides on Tuesday, opponents and supporters of same-sex marriage both have said they are ready to appeal the decision all the way to the U.S. Supreme Court.
The three-judge 9th Circuit panel plans to release its decision at about 10 a.m. (1 p.m. EDT) on Tuesday. Even if the 9th circuit finds that gay marriage is legal in California, judges likely will put their ruling on hold pending further appeal.
The losing side could ask a larger 9th Circuit panel to hear the matter, or decide to appeal directly to the Supreme Court.
About 40 of the 50 U.S. states had outlawed gay marriage before a California state court ruled in 2008 that a ban was unconstitutional, leading to a summer of gay marriages. But California voters that November by decided to change the state constitution to limit marriage to a man and woman.
It provoked some gay rights activists to take a matter that had been waged on a state-by-state basis to federal court, essentially staking the entire agenda on one case. Republican Ted Olson and Democrat David Boies - attorneys who represented George W. Bush and Al Gore, respectively, in the legal case that decided the 2000 presidential election - joined forces to take on Proposition 8 in court.
The U.S. Supreme Court is seen as a more conservative body than the lower courts that have been considering the case. Should the high court eventually decide to hear the case, much may depend on Anthony Kennedy, a Republican-appointed justice who has written important pro-gay rights decisions but has not explicitly endorsed gay marriage.
Six states - New York, Massachusetts, Connecticut, Vermont, New Hampshire and Iowa - allow gay marriage, as does Washington, D.C.
In addition, New Jersey and Washington state are considering legislation to legalize same-sex marriage, and gay rights activists in Maine say they plan to bring the issue to voters in a referendum in that state.
(Reporting By Peter Henderson and Dan Levine; Editing by Cynthia Osterman)