The U.S. Supreme Court this week takes its first serious look ever at the issue of same-sex marriage, considering two cases that raise a fundamental issue: does the Constitution's guarantee of equal protection allow legal distinctions between same-sex couples and those of the opposite sex?

The greatest potential for a ruling with nationwide implications comes in a case from California, to be argued Tuesday, brought by proponents of Proposition 8. The following day, the court will hear a separate case challenging the Defense of Marriage Act, which prevents the federal government from recognizing same-sex marriages in states where they are legal.

NBC's Pete Williams joins The Daily Rundown for a preview of the upcoming legal battle over same-sex marriages.

Approved by 52 percent of California voters in 2008, Prop 8 amended the state constitution to ban same-sex marriages. It was placed on the ballot after 18,000 couples had been legally wed there.

A federal judge in San Francisco declared the ban unconstitutional, and the Court of Appeals for the Ninth Circuit upheld the ruling. Once a state grants a fundamental right like marriage, the appeals court said, it cannot later take it away, even by voter initiative.

Urging the Supreme Court to reverse those rulings, Prop 8's defenders argue that the state has a legitimate reason for treating same-sex couples differently: only couples consisting of a man and a woman can produce children.

"This indisputable difference between same-sex and opposite-sex relationships demonstrates that Proposition 8 is constitutional, for the Constitution requires only that a state treat similarly situated persons similarly," they argue in their legal brief filed with the court.

Paul Katami and Jeff Zarrillo, one of two gay couples fighting to strike down California's ban on gay marriage, will have their case heard Tuesday at the Supreme Court.

"California has simply reserved a special form of recognition and support to those relationships that have long been thought to uniquely further vital societal interests," they say.

What's more, Prop 8's supporters argue, marriage is such an important institution that the courts should proceed with caution before changing the rules. "It is plainly reasonable for the people of California to be concerned about the potential consequences of such a profound redefinition of a bedrock social institution."

The case began three years ago when a gay couple, Jeff Zarillo and Paul Katami in Los Angeles, along with a lesbian couple from the Bay area, Kris Perry and Sandy Stier, filed a lawsuit to challenge Prop 8. Their cause took on added notoriety when two of the nation's most prominent lawyers, conservative Ted Olson and liberal David Boies, agreed to take the case.

Olsen and Boies have been on opposite sides more often than not, most notably in the battle over counting disputed ballots from Florida in the presidential election of 2000.

They agree with the Prop 8 proponents on one issue. "Marriage is a unique, venerable, and essential institution."

The two couples, they say, "simply want to be part of it."

They argue that Prop 8 cannot meet the legal test required of a law that discriminates -- having a legitimate government purpose. Its proponents, they argue, "have never identified a single harm that they, or anyone else, would suffer as a result of allowing gay men and lesbians to marry."

Banning same-sex marriage "does not increase the likelihood that opposite-sex couples capable of procreating will decide to get married, nor would permitting gay men and lesbians to marry decrease that likelihood," they say.

President Obama, after first saying that marriage was an issue for the states to decide on their own, changed course last year. As a result, his administration's Justice Department has weighed in supporting the two couples.

"When it comes to marriage, the basic principle that America is founded on -- the idea that we're all created equal -- applies to everybody regardless of sexual orientation," Obama said at a White House news conference earlier this month.

If the Supreme Court invalidates Prop 8 based on the reasoning of the federal appeals court, the ruling would affect only California, permitting marriage for same-sex couples to resume there. No other state has granted and then withdrawn the marriage right for gay couples. But Olson and Boies urge the justices to go further and rule that because such an essential right is at stake, no state can refuse to permit same-sex couples to get married.

Thea Spyer describes her 44-year relationship with same-sex spouse Edie Windsor and how Windsor's death inspired her to fight for gay marriage rights in a case that will be heard in the Supreme Court on Wednesday.

The Supreme Court could reverse the lower courts and uphold Prop 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 court also gave itself a way out of ruling on the merits of the California case. It directed lawyers for both sides to address an unusual aspect of the controversy.

After voters approved Prop 8, California officials declined to defend it, and the legal battle was picked up by the initiative's backers. The Supreme Court must decide whether they had the legal authority to stand in for the state and pursue the battle.

The case has attracted an unusually large number of friend-of-the-court briefs. Among those filing in support of Prop 8 are the US Conference of Catholic Bishops and 20 of the 38 states that forbid same-sex marriage either by statute or constitutional amendment.

On the other side, 130 prominent conservatives urge the court to strike Prop 8 down, including former Republican Party Chairman Kenneth Mehlman, actor Clint Eastwood, and former Homeland Security Secretary Tom Ridge. Among major corporations taking the same position are Alcoa, Apple, Nike, Verizon, and Xerox.

The court will decide the case sometime before its term ends in late June.

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