Starting Tuesday, the justices will take up the issue of same-sex marriage, which includes California's Proposition 8.
The court could uphold Prop. 8 or strike it down and apply a broad ruling invalidating gay marriage bans in roughly 40 states. But the justices could also issue a ruling that applies only to California or to the nine states that allow domestic partnerships.
"When we talk about fundamental rights as it relates to the Constitution, we are talking about those rights that we as a nation designated as being some of the most sacred of all the rights we can have. And 14 times, the United States Supreme Court has described marriage as a fundamental right," said California Attorney General Kamala Harris.
People have been lined up outside the court since last Thursday to get a seat. The case is a four-year battle over the traditional concept of marriage versus the rights of same-sex couples.
Locally, supporters of marriage equality rallied at Los Angeles City Hall on Sunday. They're hopeful the U.S. Supreme Court will declare Prop. 8 unconstitutional. Voters passed the ban on same-sex marriage in November 2008.
Jeff Zarrillo and his partner Paul Katami of Burbank are one of two couples suing to overturn the ban. They'll attend the hearing Tuesday.
"It's an emotional experience, I mean, to try to explain to someone you have to validate your life in a courtroom," said Katami.
The court will also hear arguments Wednesday on the 1996 federal Defense of Marriage Act, which forbids the recognition of same-sex marriages nationwide and bars married gay and lesbian couples from receiving federal benefits.
Democratic Sen. Claire McCaskill of Missouri announced her support for same-sex marriage on Monday and Ohio Sen. Rob Portman last week became the first Republican senator to show his support for the issue.
A recent poll indicates the number of Americans who support gay marriage has grown to 53 percent, while 44 percent still remain opposed.
If Prop. 8 is upheld, opponents could put the issue to another vote in California. Recent polls show support for repealing the ban. The high court is expected to decide the case by June.
Opponents of same-sex marriage contend that there is a fundamental difference between same-sex couples and opposite-sex couples.
"This is not an issue of civil rights or…discrimination," said Randy Thomasson of savemarriage.com. "Marriage is for a man and a woman. That's not only historical, it's natural. You need to have a man and a woman to have a baby."