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(NBC NEWS) -- Today the US Supreme Court hears the first of two cases that could change how America deals with same-sex marriage.

Today's case will decide whether California's voter-approved ban - known as Proposition 8 - is unconstitutional.

People on both sides of the issue have been camped out for days to get one of the few seats inside.

The first campers arrived here last Thursday - five days ago. When the court heard a landmark challenge to the new health law - the line only started three days early - giving us some idea for many people just how important this case is. "It's ok to disagree with us about issues but we still need to be recognized as part of the fabric of this country," said Gay Rights Advocate Frank Colasonti.

Experts say it'll be hard for the Court to ignore a huge shift in public opinion. Twenty years ago, most of the country opposed same-sex marriage. "The fact that the majority of Americans support marriage for gay and lesbian couples is something that is going to be in the back of justices minds," said Elizabeth Wydra, of the Constitutional Accountability Center.

Two California couples are challenging the state's 2008 voter-approved ban.

Opponents say it violates the 14th amendment. "We want to feel the same sense of security that married couples want to have," said Gay Rights Advocate Sandy Stier, "We think it's worth fighting for."

Supporters argue traditional marriage must be protected. "The institution of marriage and marriage laws are designed to attach mothers and fathers to each other and to the children that they may create and raise in the best environment," said Austin Nimocks, Lawyer for Prop 8

9 states plus Washington DC allow same sex marriage, 38 states don't.

President Obama has changed his mind, and so have some Republicans. "If you believe the government shouldn't be involved in personal lives - which most Republicans and most Conservatives DO - then I don't see how it's contradictory," said Alex Lundry, Republican pollster.

"We're here for them to get married," said Child of gay parents, Leo Crampton.

Last night's vigil - just the beginning of two days of arguments that will determine how American defines marriage, and for whom.

It's with another election in less than two years. It's not just about the law here. Republicans say changing views on same-sex marriage could open up donations in traditionally liberal states like California and New York.

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