WASHINGTON - For more than two decades, the defining battles within the Supreme Court over social and moral controversies have been fought between two Roman Catholics appointed by President Ronald Reagan.

Justice Antonin Scalia believes the law can and should enforce moral standards, including criminal bans on abortion and on "homosexual conduct" that many "believe to be immoral and destructive."

Justice Anthony M. Kennedy is a libertarian conservative who believes the Constitution protects the freedom of individuals to "make personal decisions relating to marriage, procreation, contraception, family relationships, child rearing and education."

Now the ideological fight between the conservative giants is set for another round. The two 76-year-olds are likely to be on opposite sides this spring when the justices meet to decide whether the government can refuse marriage and federal benefits to gays and lesbians.

The two have much in common. Born in 1936, they graduated from high school in the early 1950s and excelled at Harvard Law School, where they were a year apart. They were Republicans who rose through the legal ranks. They agree on much. Both voted to strike down President Obama's health care law as government overreach. Scalia joined Kennedy's majority opinion in the Citizens United case that freed corporate and union spending on political ads.

But Kennedy, the libertarian, and Scalia, the social conservative, clash fiercely over the court's role in deciding moral controversies.

The two split 20 years ago when the court's conservative bloc was poised to overturn Roe v. Wade, the ruling that legalized abortion. Although personally opposed to abortion, Kennedy switched sides in spring 1992 and cast a crucial vote to uphold a woman's right to have an abortion. "Our obligation is to define the liberty of all, not to mandate our own moral code," Kennedy wrote.

In the past, Scalia has accused Kennedy of having "signed on to the so-called homosexual agenda." Scalia is likely to have the votes of fellow conservatives Clarence Thomas, Samuel Alito and probably Chief Justice John G. Roberts Jr. to uphold state and federal laws that exclude gays from marriage.

But Kennedy has the much stronger hand. He ranks third in seniority after the chief justice and Scalia, and he has four liberal justices on his left. Because the senior member of the majority decides who writes the opinion, Kennedy could decide who writes the opinions if he votes with the liberals. And he could take the assignment for himself.

His past writings provide clues as to how he will see the issue.

In a New York case, the justices will decide whether the federal government can deny legally married same-sex couples the benefits that go with marriage. They include filing joint tax returns and receiving survivors benefits from Social Security.

Gay rights advocates challenged this exclusion in the Defense of Marriage Act as discriminatory, and they have won rulings from judges in New England, New York and Northern California.

Kennedy is likely to agree with the challengers, and he explained why in 1996, the same year Congress passed DOMA. The court then faced a Colorado voter measure that repealed gay rights ordinances in several cities. Kennedy spoke for the court in striking it down. The measure was "born of animosity" toward gays, he said, and the Constitution "prohibits laws singling out a certain class of citizens for disfavored legal status or general hardships."

If Kennedy and the court strike down the federal benefits provision of DOMA, it would be a major victory for gay rights, but it would not affect the 41 states where same-sex marriage is forbidden.

The California case on Proposition 8 could be far more significant because it involves the right to marry. Ted Olson and David Boies, the attorneys who led the challenge, plan to argue broadly that marriage is a fundamental right and that excluding gay couples from marriage denies them equal protection.

A Kennedy-Scalia clash from a decade ago gives a preview. When two gay men challenged a Texas anti-sodomy law, Kennedy wrote a glowing opinion taking their side. "They are entitled to respect for their private lives," he said, and "the state cannot demean" them by treating them as second-class citizens.

In a moment of high drama, Kennedy gave a professorial reading of his opinion on the last day of the court term in 2003. When he finished, Scalia's voice cut through the room as he delivered an angry dissent.

Kennedy's opinion left the laws against same-sex marriage "on pretty shaky grounds," Scalia said at the time. "If moral disapprobation of homosexual conduct is 'no legitimate state interest' ... what justification could there possibly be for denying the benefits of marriage to homosexual couples?"

That's the question now before the court in the California case.

By David G. SavageTribune Washington Bureau WASHINGTON - For more than two decades, the defining battles within the Supreme Court over social and moral controversies have been fought between two Roman Catholics appointed by President Ronald Reagan.Justice Antonin Scalia believes the law can and should enforce moral standards, including criminal bans on abortion and on "homosexual conduct" that many "believe to be immoral and destructive."Justice Anthony M. Kennedy is a libertarian conservative who believes the Constitution protects the freedom of individuals to "make personal decisions relating to marriage, procreation, contraception, family relationships, child rearing and education."Now the ideological fight between the conservative giants is set for another round. The two 76-year-olds are likely to be on opposite sides this spring when the justices meet to decide whether the government can re