"He is a California establishment Republican with moderately libertarian instincts," Stanford University law professor Pamela Karlan said of Kennedy. "He travels in circles where he has met and likes lots of gay people."
Based on Kennedy's past opinions, Karlan is confident that if the Supreme Court takes up the issue of California's same-sex marriage ban, "it means Prop 8 is going down to defeat," she said. "There is no way he will take it to reinstate" the ban.
Not all court observers share her prediction, but the uncertainty about how Kennedy might vote may, by itself, be enough to deter the high court from hearing an appeal of the decision by a panel of the San Francisco-based U.S. Court of Appeals for the Ninth Circuit.
A tough choice
If an appeal reaches the court, the four most conservative justices would face a tough choice: Vote to hear the case and run the risk that Kennedy would side with the more liberal justices 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.
The justice at the center of the speculation grew up in a Catholic family in Sacramento, where his father was a lawyer and lobbyist in the Legislature. Family friends included then-Gov. Earl Warren. As a Harvard law student, Kennedy visited the Supreme Court to meet with Warren, who was by then chief justice.
Kennedy, who has been on the court since 1988, has reflected at times both styles of Republicanism: the conservatism and respect for states' rights of President Ronald Reagan, who appointed him, as well as Warren's devotion to civil rights and fair treatment.
Two years ago, he wrote the much-disputed 5-4 opinion in the Citizens United case that said corporations and unions had a free-speech right to spend unlimited sums on election campaigns. But he also wrote a 5-4 opinion that struck down as cruel and unusual punishment the laws in Florida and elsewhere under which juvenile offenders were sent to prison for life for crimes that did not involve a murder.
Eight years ago, he wrote the decision that declared unconstitutional laws in Texas and elsewhere that made gays subject to arrest for "deviate" sexual conduct. "The state cannot demean" same-sex couples by making their intimate, private conduct into a crime, Kennedy said.
In 1996, he wrote an opinion in a Colorado case called Romer v. Evans that formed the basis for Tuesday's Ninth 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 Ninth Circuit decision, Judge Stephen Reinhardt did not say that gays had a right to marry as a matter of equal treatment. Instead, he focused on same-sex marriage in California and, citing Romer, repeated Kennedy's view that voters could not take away the rights gays had briefly won.
Kennedy sits in the middle of two ideological blocs. The four conservatives - Chief Justice John G. Roberts Jr. and Justices Scalia, Clarence Thomas, and Samuel A. Alito Jr. - are likely to oppose the Ninth Circuit ruling on the grounds that judges should not force such a change in state law. The four liberals - Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor, and Elena Kagan - are likely to support the appeals panel ruling as a matter of equal treatment.
Chapman University law professor John Eastman said conservatives had not given up on Kennedy: "I think Justice Kennedy will ask, Do we want to put a stake in the heart of an institution, marriage, that has done so much for society?"
Professor Erwin Chemerinsky, dean of UC Irvine Law School, believes Kennedy would write a broader opinion that undercuts other state laws banning same-sex marriage.
"This," he said, "is a court that wants to have the last word on major legal issues."