WASHINGTON - The Supreme Court announced Friday that it would enter the national debate over same-sex marriage for the first time, agreeing to hear a pair of cases challenging state and federal laws that define marriage to include only unions of a man and a woman.

One of the cases, from California, could establish or reject a constitutional right to same-sex marriage. Another case, from New York, challenges a federal law that requires the federal government to deny benefits to same-sex couples married in states that allow such unions.

The court's move comes against the backdrop of a rapid shift in public attitudes about same-sex marriage, with recent polls indicating that a majority of Americans support allowing such unions. After last month's elections, the number of states authorizing same-sex marriage increased by half, to nine.

The court's docket is now crowded with cases about the meaning of equality, with the new cases joining ones on affirmative action in higher education and the future of the Voting Rights Act of 1965. Decisions in all of those cases are expected by June.

The new California case was filed in 2009 by Theodore B. Olson and David Boies, two lawyers who were on opposite sides in the Supreme Court's decision in Bush vs. Gore, which settled the 2000 presidential election. The suit argued that California's voters had violated the federal Constitution the previous year when they overrode a decision of the state's Supreme Court allowing same-sex marriages.

A federal judge in San Francisco agreed, issuing a broad decision that said the Constitution required the state to allow same-sex couples to marry. The decision has been stayed.

A divided panel of the 9th U.S. Circuit Court of Appeals affirmed the decision. But the majority relied on narrower grounds that seemed calculated to avoid Supreme Court review or, at least, attract the vote of the court's presumed swing member, Justice Anthony Kennedy.

Judge Stephen Reinhardt, writing for the majority, relied heavily on a 1996 majority opinion from Kennedy in a case which struck down a Colorado constitutional amendment that had banned the passage of laws protecting gay men and lesbians. The voter initiative in California, known as Proposition 8, had done something similar, Reinhardt wrote.

That reasoning, he added, meant that the ruling was confined to California. "We do not doubt the importance of the more general questions presented to us concerning the rights of same-sex couples to marry," he wrote. "For now, it suffices to conclude that the people of California may not, consistent with the federal Constitution, add to their state Constitution a provision that has no more practical effect than to strip gays and lesbians of their right to use the official designation that the state and society give to committed relationships, thereby adversely affecting the status and dignity of the members of a disfavored class."

The high court has several options in reviewing the decision. It could reverse it, leaving California's ban on same-sex marriage in place. It could affirm on the narrower theory, which would allow same-sex marriage in California but not require it elsewhere. Or it could address the broader question of whether the Constitution requires states to allow such marriages -- banned by 31 state constitutions.

The second case challenges a part of the Defense of Marriage Act of 1996. Section 3 of the law defines marriage as between only a man and a woman for purposes of more than 1,000 federal laws and programs.

The case concerns two New York City women, Edith Windsor and Thea Clara Spyer, who were married in 2007 in Canada. Spyer died in 2009, and Windsor inherited her property. The law did not allow the IRS to treat Windsor as a surviving spouse, and she faced a tax bill of $363,000 that a spouse in an opposite-sex marriage would not have had to pay.

Windsor sued, and in October the federal appeals court in New York struck down the law. It also was the first decision from a federal appeals court to say that laws treating same-sex couples differently must be subjected to heightened judicial scrutiny.

Minnesota same-sex marriage advocates are encouraged by the move. They defeated a proposal similar to Proposition 8 in November and are beginning a push in the Legislature to legalize same-sex marriage in Minnesota.

"It goes to show that the conversation is robust now on this topic," said Richard Carlbom, campaign manager for Minnesotans United for All Families, the lead group that defeated the amendment. "We are happy the courts are going to take it up."

While the court's eventual decision could create a range of possible outcomes for Minnesota, same-sex marriage advocates believe momentum and public opinion is already tilting in their favor.

Same-sex marriage opponents are just as hopeful. They interpret the court's decision as a sign justices disagree with a lower court ruling against Proposition 8. They believe the justices will ultimately strengthen states' -- and voters' -- rights to ban same-sex marriage.

"Such a reversal would affirm that the power to make basic policy decisions, like the definition of marriage, rightly belongs to the people, not judges," said Chuck Darrell, a spokesman for the Minnesota for Marriage, the group that unsuccessfully pushed the marriage amendment this year. "It's a significant move."

Staff writer Baird Helgeson contributed to this report.