When the U.S. Supreme Court hears arguments Tuesday and Wednesday about same-sex marriage rights, Justice Anthony Kennedy will be scrutinized for his every word, sigh and twitch.

Since the earliest whispers of the issue reaching the nation's high court, it has been Kennedy's vote that both sides figured they must court most. He is considered the center of a court otherwise equally divided between liberals and conservatives.

And if there is an issue likely to splinter liberals and conservatives, it is gay marriage.

"To look at the larger question here, whether the court is ready (to decide the central issues in same-sex marriage), the question is - is Kennedy?" said John Eastman, a conservative law professor and head of the National Organization for Marriage.

The Supreme Court on Tuesday will hear the legal challenge to California's Proposition 8, the 2008 voter-approved ban on same-sex marriage. The justices are reviewing a federal appeals court ruling that declared the law unconstitutional because it stripped away a previous right of same-sex couples to marry in the Golden State.

On Wednesday, the court will hear arguments in the challenge to the 1996 federal Defense of Marriage Act, which denies federal benefits, from Social Security to tax status, to gay and lesbian couples. A federal appeals court struck down the law last year in a New York case, concluding the federal government cannot deny benefits to same-sex couples in states that legalize gay marriage.

For all the attention on Kennedy, legal experts say there are so many scenarios for the court in both cases, from issuing a sweeping decision on gay marriage to resolving the cases on narrow procedural grounds, that all nine of the justices bear watching. Four of the court's members, Chief Justice John Roberts and Justices Samuel Alito, Elena Kagan and Sonia Sotomayor, have no judicial precedent on gay rights thus far.

Many legal experts are tracking Roberts, who surprised conservatives last year when he cast the deciding vote to uphold President Barack Obama's health-care law. But for the most part, the justices are expected to split along traditional lines.

"Are there any wild cards?" said Jane Schachter, a Stanford University law professor. "I doubt it, among that group."

Kennedy's role in the two cases surpasses ideology. He has authored the two most recent Supreme Court decisions supporting gay rights, perhaps the key precedents in arguments over whether gay marriage bans are constitutional.

In Lawrence v. Texas, Kennedy in 2003 wrote the 6-3 majority opinion striking down a Texas anti-sodomy criminal statute, saying the law targeted gays' private conduct and served no legitimate government purpose. To much notice, Justice Antonin Scalia dissented, warning that Kennedy's reasoning could eventually be used to argue for same-sex marriage.

And in 1996, Kennedy wrote the majority opinion in Romer v. Evans, which found unconstitutional a Colorado law that denied anti-discrimination protections to gays and lesbians.

In briefs filed in this week's marriage cases, lawyers for same-sex couples and the Obama administration rely heavily on Romer and Lawrence to urge the Supreme Court to find gays and lesbians should have a legal right to marry. And therefore they rely heavily on Kennedy's precedent.

"Romer and Lawrence were the only two cases advancing gay rights in American history, and they were both written by Anthony Kennedy," said Erwin Chemerinsky, dean of the law school at the University of California, Irvine.

While Romer and Lawrence are being heavily debated for their potential influence on the same-sex marriage cases, there is another Supreme Court precedent at the heart of the legal arguments. It is the court's 1967 ruling in Loving v. Virginia, which found bans on interracial marriage unconstitutional.

Lawyers for gay marriage opponents have argued in their briefs that the same-sex marriage cases are distinct from Lawrence, Romer and Loving. In fact, they argue that Loving is different because it involved heterosexual couples who could procreate, one of the key reasons they say same-sex marriage bans are justified.

"The fundamental right to marry recognized by this court in Loving ... was tied to the unique procreative capacity of opposite-sex unions," former U.S. Attorney General Edwin Meese III wrote in one brief.

To legal experts, the Supreme Court's reliance on those key, cases may depend on whether the justices are ready to resolve the gay marriage controversy now, with so many states still bitterly divided on the issue.

"A common refrain is that the Supreme Court should only issue decisions that are durable," said Marc Spindelman, an Ohio State University law professor. "It looks like the winds of change are moving in one direction now. But it still might be too soon for the court."

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