Discuss as:

On gay marriage, Supreme Court could rule narrowly -- or issue sweeping decision

The Supreme Court will take its first serious look at same-sex marriages. NBC's Pete Williams reports.

WASHINGTON -- The marriage cases the U.S. Supreme Court has agreed to hear will test an axiom about the court, one that says it prefers to move in small increments rather than big leaps.

By next June, the justices could rule narrowly on the constitutionality of allowing same-sex couples to get married. Or they could announce a sweeping ruling that would apply nationwide and remain the law of the land for years to come.

"We have no idea if they will ultimately reach the broader issues about gay people and the fundamental right to marry," says Mary Bonauto of Gay and Lesbian Advocates and Defenders, a veteran of the legal battles over gay rights.

It's possible the court will find a way to avoid the central issue both cases raise: Does the Constitution's guarantee of equal protection allow the state and federal governments to make legal distinctions between same-sex couples and those of the opposite sex?

The court agreed Friday to consider challenges to a law that bars the federal government from recognizing same-sex marriages in the states where they are legal and to California's Proposition 8, the voter-approved initiative that put a stop to marriage in that state for gay couples.

Both cases invite the potential for far-reaching decisions, says Tom Goldstein, a Washington, D.C., lawyer who argues before the court and publishes the SCOTUSblog website.

"This is a monumental action by the Supreme Court, because we know they're going to say something about gay marriage for the first time ever. They may not decide there's a constitutional right to it, but this will be the building block, in one direction or the other, for recognizing or rejecting that right," Goldstein says.

The greatest potential for a ruling with nationwide implications comes in the California case.  Proposition 8, approved by 52 percent of California voters in 2008, amended the state constitution to ban same-sex marriages. It went into effect after 18,000 couples had been legally married there.

A federal judge in San Francisco declared the ban unconstitutional, and the Court of Appeals for the Ninth Circuit upheld the ruling. But it based its decision on narrower grounds that apply only to California. 

Once a state grants a fundamental right like marriage, the appeals court said, it cannot later take it away. 

"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,” wrote Judge Stephen Reinhardt.

If the Supreme Court affirms that decision, the ruling would affect only California, permitting marriage for same-sex couples to resume.  No other state has granted and then withdrawn the marriage right for gay couples.

But the justices could go further and decide whether any state can refuse to permit same-sex couples to get married. 

"The question whether the states may discriminate against gay men and lesbians in the provision of marriage licenses is the defining civil rights issue of our time," argues Ted Olson, the Washington, D.C., lawyer representing two gay couples who challenged the California measure.

The Supreme Court could also reverse the lower courts and uphold Proposition 8 as a legitimate exercise of the people's right to amend their state constitution, an outcome urged by the ballot measure's backers.

The appeals court decision, they argue, "threatens to short-circuit further democratic deliberations regarding official recognition of same-sex relationships."

The Supreme Court also gave itself a way out of reaching the merits of the California case.  It directed lawyers for both sides to address an unusual aspect of the controversy.

After voters approved the initiative, which became part of the state constitution, California officials declined to defend it and the legal battle was picked up by Prop 8's backers.  The Supreme Court wants to know if they had the legal authority to stand in for the state and carry on that battle.

The second case the Supreme Court agreed on Friday to hear is a challenge to a federal law passed in 1996 and signed by President Clinton.  The Defense of Marriage Act, known as DOMA, bars federal agencies from recognizing same-sex marriages in the states where they are legal under state law.

The court took up a lawsuit filed by a New York woman, Edie Windsor, who lived for more than 40 years with her partner, Thea Spyer, marrying her in Canada in 2007.

But when Spyer died two years later, leaving Windsor the estate, the IRS sent a tax bill for $363,000, because the federal government did not recognize their marriage.  The surviving spouse of a traditional marriage is generally not required to pay federal estate taxes.

“It was the injustice, I think, ultimately,” Windsor says.  “I couldn't believe that they were making a stranger of this person I lived with and loved for 43-something years."

A decision striking down DOMA would not, by itself, require states to allow same-sex marriages.  But the federal government would be required to recognize those marriages in the states where they are legal.

In that case, too, the court provided itself an off-ramp.  After first defending the law, the Obama administration concluded last year that DOMA was unconstitutional.  House Republicans picked up the legal defense. The Supreme Court asked lawyers for both sides to address whether the House Republicans have the legal right to carry on the appeal.

Both cases will be argued in March, and a decision probably will not come before late in June.

More content from NBCNews.com:

Follow US news from NBCNews.com on Twitter and Facebook