WASHINGTON The Supreme Court has nine justices, but if the constitutional fight over same-sex marriage reaches them this year, the decision will probably come down to just one: a California Republican and Reagan-era conservative who has nonetheless written the court's two leading gay rights opinions.

Justice Anthony Kennedy, 75, often holds the court's deciding vote on the major issues that divide its liberals and conservatives. More often than not, that vote has swung the court to the right. But on gay rights, Kennedy has been anything but a "culture wars" conservative.

One of his opinions lauded the intimacy between same-sex couples and demanded "respect for their private lives," provoking Justice Antonin Scalia to accuse him of having "signed on to the so-called homosexual agenda."

To hear or not to hear

The uncertainty about how Kennedy would vote may, by itself, be enough to deter the high court from hearing an appeal of the decision by the U.S. 9th Circuit Court of Appeals. Four justices must vote for the court to consider a case, but a majority is needed to issue a ruling.

When an appeal reaches the high court, the four most conservative justices will face a tough choice: Vote to have the court hear the case and run the risk that Kennedy would side with the more liberal justices to go beyond the 9th Circuit decision and establish a nationwide right to same-sex marriage. Or turn the case aside, leaving same-sex marriage intact in California but setting no national precedent.

Related Colorado case

In 1996, Kennedy wrote an opinion in a Colorado case called Romer v. Evans that formed the basis for Tuesday's 9th Circuit decision striking down Proposition 8.

Colorado voters had approved an initiative that stripped gays and lesbians of civil rights protections under state and local ordinances. Kennedy said the law could not stand because it was "born of animosity" toward homosexuals and took away their hard-won legal rights.

In Tuesday's decision, Judge Stephen Reinhardt of Los Angeles repeated Kennedy's view that voters could not take away the rights gays had briefly won.

"Prop. 8 singles out same-sex couples for unequal treatment by taking away from them alone the right to marry," Reinhardt wrote, citing Romer v. Evans.

"Both sides will be nervous," said Michael Dorf, a Cornell University law professor who has clerked for Reinhardt and Kennedy. The California-only approach taken by Reinhardt would allow the high court to pass up the case, but he and others predict the justices will hear it. "This legalizes same-sex marriage in the biggest state. That's a big deal in itself," Dorf said.