On Wednesday, the court will consider the Defense of Marriage Act of 1996, which prohibits gay and lesbian married couples from receiving the same federal benefits afforded heterosexual couples.
Public interest is running so high that people began lining up for seats late last week. Rulings in both cases are expected by late June.
It is the Proposition 8 case, filed by two California couples, that could yield a ruling that squarely addresses the constitutionality of same-sex marriage. Last week, plaintiffs' lawyer David Boies said the high court will be asked to rule that no state can ban same-sex marriage.
"We believe ... you need to establish marriage equality nationally," Boies said in a conference call. "We think the time is right for that broader decision."
One thing's sure: The justices themselves are often mindful of matters such as timing and the national mood. As a case in point, legal scholars note that Ruth Bader Ginsburg, a stalwart liberal, believes Roe v. Wade, decided in 1973, actually damaged the cause of abortion rights by pushing the country too far, too fast.
At present, the vast majority of states explicitly limit marriage to opposite-sex couples. Nine states and the District of Columbia allow same-sex marriages.
The Proposition 8 arguments are unusual because California has chosen not to defend the measure. Instead, the court will hear from Charles Cooper, a former Justice Department official during the Reagan administration who represents ProtectMarriage.com, a coalition of groups that oppose same-sex marriage.
The central purpose of marriage, as understood throughout recorded history, is to foster "potentially procreative sexual relationships for enduring, stable unions for the sake of responsibly producing and raising the next generation," Cooper wrote in his brief before the court. In prior rulings, the court has recognized that marriage meant the union of "one man and one woman," he said.
The long-term implications of redefining marriage are profound, and it is "hardly surprising" that voters in most states have decided not to do so, Cooper said. "Perhaps, their views will change as experience with same-sex marriage in other states matures," Cooper said. "And perhaps not. But whether marriage should be redefined is for the people to decide."
Countering that view, Boies and his co-counsel, Ted Olson, have argued that discrimination is not permissible, even if it is customary.
"If a history of discrimination were sufficient to justify its perpetual existence ... our public schools, drinking fountains and swimming pools would still be segregated by race, our government workplaces and military institutions would still be largely off-limits to one sex - and to gays and lesbians - and marriage would still be unattainable for interracial couples," their brief said.
The Equal Protection Clause of the U.S. Constitution did not tolerate those discriminatory practices, and it similarly does not tolerate "the permanent exclusion of gay men and lesbians from the most important relation in life."
President Obama has also asked the court to strike down Proposition 8. "Tradition, no matter how long established, cannot by itself justify a discriminatory law," the administration said in a friend-of-the-court brief.
But the president stopped short of insisting that same-sex marriages should be a constitutional right nationwide. Instead, the administration said bans should not be allowed in states that have already granted same-sex couples almost all the rights of marriage, such as civil unions and domestic partnerships.
California is one of eight such states, the brief said. The others are New Jersey, Delaware, Hawaii, Illinois, Nevada, Oregon, and Rhode Island. (Last week, Colorado's governor signed into law a measure allowing civil unions.)
On Wednesday, the court will consider Edie Windsor's challenge to the Defense of Marriage Act, or DOMA, which was passed overwhelmingly by Congress in 1996 and signed into law by President Bill Clinton. (Then-Sen. Joseph Biden was among those who voted for DOMA.) The law denies more than 1,100 federal benefits to same-sex couples who have been legally married.
Windsor, 83, married Thea Spyer in 2007 after they had been a couple for four decades. When Spyer died two years later, the IRS did not recognize the validity of their marriage, which meant Windsor owed $363,053 in estate taxes. Windsor's lawyers are asking the high court to not only strike down DOMA, but also to treat gays and lesbians as a protected class under the Constitution, as the court has done with women and minorities.