The Post’s Robert Barnes stops by to fill us in on the Supreme Court’s decision to rule on some aspects of same-sex marriage during its current term. (The Fold/The Washington Post)

The Supreme Court put itself at the center of the nation’s debate over whether gay couples have the same fundamental right to marry as heterosexuals, agreeing Friday to review state and federal efforts to preserve a traditional definition of husband and wife.

In agreeing to hear cases from California and New York, the court raised the possibility of a groundbreaking constitutional decision on whether the right to marry may be limited because of sexual orientation. At the same time, the justices also will have the ability to issue narrower rulings on a subject that continues to divide the American public.

The cases will probably be heard in historic sessions at the court in late March, with decisions to come when the justices finish their work at the end of June.

The court’s first review of same-sex marriage comes at a fast-moving but unsettled time in the nation’s consideration of gay rights. Last month brought Election Day victories for same-sex marriage supporters in three states, including Maryland, and the reelection of President Obama, the first chief executive to endorse the right of gays to marry.

But the vast majority of states ban such unions, and 31 of them have amended their constitutions to enshrine the traditional definition of heterosexual marriage.

Same-sex marriage status in the U.S., state-by-state

The acceptance of the same-sex marriage cases heralds a landmark term for the court on civil rights issues; it had already agreed to consider whether racial preferences may play a role in college admissions and the future of a key part of the Voting Rights Act of 1965 enacted to protect minorities.

The court was almost obliged to review the 1996 Defense of Marriage Act (DOMA), which defines marriage as between a man and a woman and withholds federal benefits from same-sex couples legally married in the states where they live. The law affects things such as health insurance, taxes and medical leave.

Obama announced in 2011 that his administration would no longer defend the law against challenges that it violated the Constitution’s guarantee of equal protection. Four district courts and two courts of appeal have declared the law unconstitutional, including in the New York case, and the high court almost always weighs in on such decisions.

But the court took a bold step in agreeing to review a lower court’s ruling overturning Proposition 8, the 2008 measure in which Californians amended their state constitution to ban same-sex marriage. The referendum came after the state Supreme Court had ruled that there was a right to same-sex marriage and 18,000 couples had taken advantage of the move.

The decision to take the California case raised the possibility that the court would grapple directly with fundamental questions about the right to marry.

The acceptance of the two cases give the justices the ability to take a broad look at gay rights and marked a dramatic moment in the nation’s history.

“This is a momentous case,” said Washington lawyer Theodore Olson, a Republican former solicitor general who combined with Democratic attorney David Boies to bring the challenge to Proposition 8. “It will be an education for the American people, and we are very confident that the outcome of this case will be to support the marriage rights of our gay and lesbian brothers and sisters.’’

Those opposed to same-sex marriage took heart that the court agreed to hear the Proposition 8 challenge, instead of letting stand the lower court decision overturning it.

“We believe it is a strong signal that the court will reverse the lower courts and uphold Proposition 8,” said John Eastman, chairman of the board of the National Organization for Marriage. “That is the right outcome based on the law and based on the principle that voters hold the ultimate power over basic policy judgments and their decisions are entitled to respect.”

The Proposition 8 case offers the court a more direct path to deciding whether the fundamental constitutional right to marry may be limited based on sexual orientation. But because of the way the U.S. Court of Appeals for the 9th Circuit decided the case, such a finding is not required.

Initially, a San Francisco federal judge had decided that gay couples may not be excluded from the right to marry that heterosexual Americans enjoy. But the 9th Circuit panel found more narrow ground in overturning Proposition 8.

When 52 percent of California voters supported Proposition 8 in 2008, amending the state constitution to validate “only a marriage between a man and a woman,” they removed from gay couples the right to marry. (The unions of those already married were not affected.)

U.S. Circuit Judge Stephen Rein­hardt said this “taking away” of a right by the majority was not allowed.

“By using their initiative power to target a minority group and withdraw a right that it possessed, without a legitimate reason for doing so, the people of California violated the Equal Protection Clause” of the U.S. Constitution, Reinhardt wrote.

He based the ruling on the Supreme Court’s 1996 decision in Romer v. Evans, which involved an amendment to the Colorado constitution that prohibited the state or local jurisdictions from outlawing discrimination against gays. The amendment was a response to local governments that had extended such protection.

Reinhardt’s decision, if allowed to stand, would apply only to same-sex marriages in California.

The court was in a tricky legal area in deciding what to do about DOMA. The Obama administration normally would defend a law passed by Congress but would prefer that this one be found unconstitutional. The Republican leadership in the House has hired Paul D. Clement, the former solicitor general who argued the challenges to the Affordable Care Act earlier this year, to defend DOMA.

From several possibilities, the court chose the case of Edith “Edie” Windsor, 83, who is represented by the American Civil Liberties Union.

Windsor had to pay $363,000 in estate taxes after her partner of 44 years died in 2009. Windsor and Thea Spyer were married in 2007 in Canada and lived in New York. If Windsor had been married to a man, she would not have owed the tax payment.

The U.S. Court of Appeals for the 2nd Circuit said that deprived Windsor of her constitutional right to equal protection. It also said the law should be subjected to a legal standard called “heightened scrutiny” because it unfairly discriminated against gay men and lesbians, the first time a court had made such a finding.

Central to the outcome of the term’s signature cases will be Justice Anthony M. Kennedy, who normally sides with the court’s conservatives but has written some of the court’s most important cases upholding gay rights. For instance, he wrote the Romer decision that the 9th Circuit used as the template for overturning Prop 8.

But some gay rights activists have worried about asking Kennedy and the court to move too far too quickly on what would be a sea change in the way Americans view marriage.

On the one hand, public opinion has changed dramatically: Two-thirds of younger Americans supported same-sex marriage in the most recent Washington Post-ABC News poll. Maryland, Washington and Maine became the first states to approve gay marriage at the polls last month. They join Connecticut, Iowa, Massachusetts, New Hampshire, New York, Vermont and the District, where same-sex marriage was approved either legislatively or by courts.

On the other hand, same-sex marriages are not allowed elsewhere in the country. In May, North Carolina became the most recent to approve a constitutional ban. Two-thirds of older Americans disapprove of the unions.

While the House Republicans defending DOMA declined to comment on Friday’s action, Edie Windsor released a statement that reflected on a changing nation.

“When Thea and I met nearly 50 years ago, we never could have dreamed that the story of our life together would be before the Supreme Court as an example of why gay married couples should be treated equally, and not like
second-class citizens,” Windsor said.

“While Thea is no longer alive, I know how proud she would have been to see this day. The truth is, I never expected any less from my country.”

The Proposition 8 case is Hollingsworth v. Perry , and the DOMA case is United States v. Windsor .

Scott Clement and Jerry Markon contributed to this report.