Washington (CNN) -- The meaning of marriage.
It's an issue that does not get more basic, yet the complexity surrounding the legal, social, and political implications of expanding that right to gays and lesbians is now squarely before the Supreme Court.
The justices launch an epic public dialogue on Tuesday when they hear oral arguments in the first of two appeals to state and federal laws restricting same-sex marriage. The second round will be on Wednesday.
However, the real challenge and drama will come when they go behind closed doors later this week and vote as a group -- at least preliminarily -- on questions presented in cases with landmark potential.
The political, social, and legal stakes of this long-simmering debate have once again put the high court at the center of national attention, a contentious encore to its summer ruling upholding the massive health care reform law championed by President Barack Obama.
The outcome in this one could have profound influence on how America defines family. The court is likely to take its time and not act before June.
The drama inside the ornate marble courtroom will be matched by heated rhetoric outside.
Hundreds of activists on both sides will hold competing rallies in front of the court. Dozens of others have waited in line in frigid temperatures and snow -- some since Thursday -- for a coveted seat.
Prop 8: California's law
On Tuesday, the justices were set to hear arguments concerning the appeal of a federal judge's decision that struck down California's Proposition 8, which defines marriage as between a man and a woman.
The overriding legal question is whether the 14th Amendment guarantee of "equal protection" prevents states from defining marriage as California has.
California voters approved it as a ballot initiate by 52-48% in November 2008, less than six months after the state Supreme Court ruled marriage was a fundamental right that must be extended to same-sex couples.
Two of the key plaintiffs are Paul Katami and Jeff Zarrillo from Burbank.They want to marry, and say the state is discriminating against them for their sexuality.
"Stigma is stigma. And discrimination is discrimination," Katami told CNN. "I think that any time there's discrimination in the country it needs to be addressed and it needs to be taken care of. And that's why we feel that anytime in our history when there's been racial discrimination or sexual discrimination of orientation or in particular marriage at this point that we always bend toward the arch of equality."
There are an estimated 120,000 legally married same-sex couples in the United States. It is legal in nine states: Washington, Iowa, Maine, Massachusetts, Connecticut, Vermont, New Hampshire, Maryland, and New York, along with the District of Columbia.
Another nine states have civil union or strong domestic partnership laws, that stop short of marriage.
The Obama administration has formally expressed support for same-sex marriage in California, weighing in on the case in a brief last month. Obama, whose views on the issue have evolved over his political career to full support, said that he would vote to strike down the state's law if he sat on the court.
In what some have labeled the "nine-state strategy," the Justice Department argument is expected to center around the idea that civil union and domestic partnership laws may themselves be unconstitutional and that those states should go all the way and grant same-sex couples full marriage rights.
A politically charged issue
Other prominent politicians have expressed timely opinions in recent weeks, indicating the importance of the matter in the social context of 21st century American politics.
Hillary Clinton, the former secretary of state and possible 2016 Democratic presidential candidate, recorded a video for a gay rights group indicating her support.
Republicans have described cracks appearing in their party's long-held opposition to same-sex marriage.
"There is no putting this genie back in the bottle. It is undeniable. The shift is here and we're not going back." Republican strategist and CNN contributor Ana Navarro said on Sunday.
One prominent Republican, Sen. Rob Portman of Ohio, endorsed it this month after his son revealed to him that he was gay.
A new national poll indicates increasing public awareness around the issue and stronger overall support for same-sex marriage specifically.
According to the CNN/ORC International survey, 57% say they have a family member or close friend who is gay or lesbian, up 12 points from 2007. Also, the number of Americans who support same-sex marriage has risen by almost the same amount over the same period - from 40% in 2007 to 53%.
In its separate argument on Wednesday, the justices will tackle the federal Defense of Marriage Act (DOMA), a 1996 law defining marriage for federal purposes as between a man and a woman.
That means legally married gay and lesbian couples are denied federal benefits and privileges -- things like tax breaks and survivor benefits.
Backers of DOMA and Proposition 8 say it should be up to the public to decide, not the courts.
"Our most fundamental right in this country is the right to vote and the right to participate in the political process, " said Austin Nimocks of the Alliance Defending Freedom, a Christian advocacy group.
"We don't need the Supreme Court to take that right away from Americans of good faith on both sides of this issue and impose its judicial solution," Nimocks said. "We need to leave this debate to the democratic process, which is working."
But California Attorney General Kamala Harris, who is arguing against Proposition 8, said voter-approved marriage bans "are simply unconstitutional." The Supreme Court has ruled more than a dozen times that marriage is a fundamental right, "and as it relates to a fundamental right, the court will hold that under the highest level of scrutiny."
By patiently letting legislatures and the voters decide the social and practical implications of same-sex marriage over the past decade, the high court is now poised to perhaps offer the final word on the tricky constitutional questions.
The split 5-4 conservative-liberal bench has the option of ruling broadly or narrowly-- perhaps taking a series of incremental cases over a period years, building political momentum and public confidence in the process.
The case heard Tuesday is Hollingsworth v. Perry (12-144), dealing with Proposition 8; and U.S. v. Windsor (12-307) comes up on Wednesday over the DOMA issue.