The San Francisco Gay Men’s Chorus sang "Give ‘Em Hope" for a revered and in some ways surprising guest who shared a California stage with them last month: Justice Anthony M. Kennedy.

Kennedy was in San Francisco for an American Bar Association meeting, but he was also there to be celebrated by the men on the risers behind him. In remarks from the stage, San Francisco’s mayor, Edwin M. Lee, thanked the justice "for upholding the Constitution and justice for all" in his majority opinion in June in United States v. Windsor, a major gay rights victory.

"Freedom is always a work in progress," Kennedy said in his own remarks, making clear that there was more work to be done.

Kennedy has emerged as the most important judicial champion of gay rights in the nation’s history, having written three landmark opinions on the subject, including this summer’s Windsor decision, which overturned a ban on federal benefits for married same-sex couples. Those rulings collectively represent a new chapter in the nation’s civil rights law, and they have cemented his legacy as a hero to the gay rights movement.

"He is the towering giant in the jurisprudence of freedom and equality for gay people," said Evan Wolfson, the president of Freedom to Marry and one of the architects of the political and legal push for same-sex marriage.

That push has taken on momentum thanks to the Windsor decision, which gay rights groups are citing in challenges to state bans on same-sex marriage. On Thursday, the Internal Revenue Service said it would implement the Windsor ruling by recognizing the unions of all lawfully married same-sex couples, including those living in states that do not allow same-sex marriage.

On Saturday, Justice Ruth Bader Ginsburg, who joined Kennedy’s majority opinion in Windsor, became the first member of the court to officiate at a same-sex wedding.

The praise now being showered on Kennedy by gay rights advocates – and the deep disappointment of conservatives – would have been hard to imagine when President Ronald Reagan nominated him to the Supreme Court in 1987. Gay rights groups were more than a little wary then.

On the federal appeals court in California, where Kennedy had served for 13 years, he heard five cases concerning gay rights. He voted against the gay rights claim every time.

"I have to say that Kennedy seems rather obtuse on important gay issues and must be counted as a likely vote against us on most matters likely to come before the Supreme Court," Arthur S. Leonard, an authority on gay rights at New York Law School, wrote in The New York Native, a newspaper that focused on gay issues.

The justice’s trajectory since then has been a product of overlapping factors, associates and observers say. His Supreme Court jurisprudence is characterized by an expansive commitment to individual liberty. He believes that American courts should consider international norms, and foreign courts have expanded gay rights. His politics, reflecting his background as a Sacramento lawyer and lobbyist, tend toward fiscal conservatism and moderate social views. And he has long had gay friends.

Michael C. Dorf, a law professor at Cornell who served as a law clerk to Kennedy, said the key to understanding his former boss was the culture of his home state.

"The way to think about his instincts is that he is fundamentally a California Republican," Dorf said. "It’s not surprising that a California Republican in 1987 would be expected to be at best an unreliable ally for gay rights groups."

In the 1980s, California Republicans, like most Americans, had deep reservations about the notion of gay equality. But there were also early stirrings of the gay rights movement in California.

Harvey Milk, a member of the San Francisco Board of Supervisors and one of the first openly gay elected officials, delivered a landmark gay rights speech in 1978 (on which the song "Give ‘Em Hope" was based). Milk was assassinated later that year.

The same year, Reagan, a former California governor preparing to run for president, helped defeat a ballot initiative that would have made it easy to fire gay teachers.

Over time, some leading California Republicans moved more on the issue than many Republicans elsewhere. Theodore B. Olson, a Northern Californian who served as the U.S. solicitor general under President George W. Bush, helped lead the fight to overturn Proposition 8, the state’s ban on same-sex marriage.

Kennedy’s three major gay rights decisions are in this tradition, Dorf said. They also hark back to a third California Republican, Chief Justice Earl Warren, another former governor, who wrote Brown v. Board of Education, the 1954 decision barring segregation in public schools.

Romer v. Evans, in 1996, struck down a Colorado constitutional amendment that had banned laws protecting gay men and lesbians. Lawrence v. Texas, in 2003, struck down laws making gay sex a crime. And in June, Kennedy wrote the Windsor decision.

In 1987, gay rights advocates could see little of this coming. Jeffrey Levi, then the executive director of the National Gay and Lesbian Task Force, testified against Kennedy at his confirmation hearings, saying that "his past opinions offer little hope to gays and lesbians challenging adverse treatment in the courts."

Levi, who now teaches health policy at George Washington University, said in an email that "there was no way to predict that Justice Kennedy would ‘evolve’ as he did (given prior opinions on gays in the military, immigration and federal employment)."

Levi drew a comparison to Dr. C. Everett Koop, whose nomination as surgeon general under Reagan was opposed by gay rights groups based on hostile statements he had made.

"He turned out to be a hero of the early fight against AIDS," Levi said of Koop.

There were, though, other ways to read the available evidence about the Kennedy nomination. As a federal judge in 1980, when he voted to uphold the discharge of Navy personnel for homosexuality, he seemed to leave the door open to further challenges.

"We recognize, as we must," he wrote for a unanimous three-judge panel, "that there is substantial academic comment which argues that the choice to engage in homosexual action is a personal decision entitled, at least in some instances, to recognition as a fundamental right and to full protection as an aspect of the individual’s right of privacy."

Seven years later, at Kennedy’s confirmation hearings, Sen. Gordon J. Humphrey, R-N.H., said that language worried him. "My goodness," he said, "you can find academic comment to justify almost anything."

Kennedy responded that he had thought it important for the service members "to know that I had considered their point of view."

In 1986, a month after the Supreme Court upheld a Georgia law that made gay sex a crime in Bowers v. Hardwick, Kennedy, not yet a justice, gave a speech at Stanford expressing reservations about the ruling. He contrasted it to a 1981 decision from the European Court of Human Rights striking down a similar law in Northern Ireland.

Seventeen years later, Kennedy cited the European court’s decision in his majority opinion in Lawrence v. Texas, which overruled Bowers.

"Its continuance as precedent demeans the lives of homosexual persons," Kennedy wrote.

Justice Harry A. Blackmun, who wrote the majority opinion in 1973 in Roe v. Wade, which established a constitutional right to abortion, warned Kennedy to expect harsh criticism when he stood up for gay rights.

"Monday’s decision took courage," Blackmun wrote to Kennedy, praising his majority opinion in 1996 in Romer v. Evans. "You undoubtedly now will receive a lot of critical and even hateful mail. I have had that experience."

Kennedy replied: "No one told us it was an easy job when we signed on."

These days, Dorf said, there is more praise than criticism, and Kennedy has joined a select group.

"What Earl Warren was to civil rights and what Ruth Bader Ginsburg was to women’s rights," he said, "Kennedy is to gay rights."

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