For justices, the issue is not just what to decide, but when to decide it. The court has been faulted for waiting too long or moving too quickly to recognize constitutional rights.
The justices did not strike down state bans on interracial marriage until 1967, 13 years after they had declared racial segregation unconstitutional. Yet in response to the growing women's rights movement, the court in 1973 struck down all the state laws restricting abortion, triggering a national "right to life" movement and drawing criticism even from some supporters that the Roe v. Wade ruling had gone too far too fast.
Now, the justices must decide whether to hear an appeal from the defenders of California's Proposition 8, the 2008 voter initiative that limited marriage to a man and a woman.
At the same session Friday, the court will sift through several appeals to decide whether legally married gay couples have a right to equal benefits under federal law. Appeals courts in Boston and New York have struck down this part of the Defense of Marriage Act, and the justices are almost certain to take up a case to resolve that question.
The Proposition 8 case, known as Hollingsworth vs. Perry, presents justices with the more profound "right to marry" question.
Opinion polls now show a majority of Americans favor marriage equality, and support for it has been growing about 4 percent per year. On Nov. 6, voters in three states - Maine, Maryland and Washington - approved same-sex marriage, bringing the total to nine states.
Does the shift in public opinion suggest the court should uphold gay marriage now, or wait for more states, perhaps a majority, to legalize it?
The defenders of Proposition 8 say their case "raises the profoundly important question of whether the ancient and vital institution of marriage should be fundamentally redefined," and in this instance, by federal judges.
A federal judge in San Francisco struck down Proposition 8 as discriminatory and irrational. In February, the 9th U.S. Circuit Court of Appeals affirmed that by a 2-1 vote, ruling the ban on gay marriage violated the Constitution's guarantee of equal protection of the laws. The majority relied heavily on a 1996 opinion by Justice Anthony M. Kennedy that had struck down an anti-gay initiative adopted by Colorado voters.
The decision on whether to hear the case could be a hard call for the court's conservatives and liberals.
Usually, the justices are inclined to vote to hear a case if they disagree with the lower court ruling. The most conservative justices - Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. - almost certainly think the 9th Circuit's ruling was dubious. Scalia, for example, says the "equal protection" clause, added to the Constitution after the Civil War, aimed to stop racial discrimination and nothing more. He often insists the justices are not authorized to give a contemporary interpretation to phrases such as "equal protection."
If Chief Justice John G. Roberts Jr. joins the other three, the conservatives would have the needed four votes to hear the Proposition 8 case.
They may hesitate. To form a majority, they would need Justice Kennedy, the author of the court's two strongest gay rights rulings. His 2003 opinion struck down a Texas anti-sodomy law and said the state could not "demean" gays by treating them as second-class citizens. Five months later, the Massachusetts Supreme Court, citing Kennedy's opinion, became the first to rule that gays and lesbians had a right to marry.
If the court were to take up the Proposition 8 case, Kennedy, 76, would be likely to control the opinion.
"If you care about history and your legacy, that must be pretty tempting, to write the court's opinion that could be the Brown v. Board of Education of the gay rights movement," said Michael J. Klarman, a Harvard legal historian, referring to the case that ordered school desegregation.
Still, the court's liberals also may hesitate. Justice Ruth Bader Ginsburg, though a leading women's rights legal advocate, has said she thought the court made a mistake in the 1970s by moving too fast to declare a national right to abortion.
If the court votes to hear the California case, it will be decided by late June. If the appeal is turned down, it means gay marriage will become law in California, due to the 9th Circuit's decision. The court may also put off a decision on the Proposition 8 case until the justices have decided on the constitutionality of the Defense of Marriage Act, also by June. The court is likely to announce whether it intends to hear the cases by Dec. 3.
Two Los Angeles law professors - Scott Cummings at the University of California, Los Angeles and Douglas NeJaime at the Loyola Law School - said the strong shift of public opinion in favor of gay marriage argues for the Supreme Court to stand aside for now. "The tide is flowing only one way. So a wait-and-see approach seems prudent at this stage," Cummings said.
But in the South, the perspective may differ.
"Tennessee and the other deeply red states are not going to (allow gay marriage) on their own, at least for another 25 years," said Suzanna Sherry, a professor at Vanderbilt Law School. "People here sincerely believe it will harm their marriage and offend God if gays are allowed to marry." She says the court has a duty to take up the issue. "If there is no rational basis for denying gays the right to marry, the court should step in and protect gays from the tyranny of the majority."
The defenders of Proposition 8, by contrast, argue the Constitution "leaves the definition of marriage in the hands of the people, to be resolved by the democratic process in each state." Lawyers for Indiana and 14 other states also urged the court to reverse the 9th Circuit's decision, which they called "radical" and "insulting" to the voters of California who sought to restore the "state's traditional definition of marriage."
Ted Olson, the conservative Washington lawyer who joined David Boies to lead the legal attack on Proposition 8, admits he is torn over whether the Supreme Court should hear the case.
"We won the case, and if they don't take it, our clients have won. They will be allowed to marry," Olson said. "But if they take the case, it could lead to a broader victory. We believe gays and lesbians have a constitutional right to be treated equally. And if it is a constitutional right, you shouldn't have to try to win at the ballot box in every state."
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