While the fall campaign was heated  and expensive, with each side spending more than $40 million  the hearing is bound to seem somewhat anticlimactic to many. The court will only hear oral arguments on Thursday, and has 90 days to come to a decision.

Image On the eve of the hearing, opponents of Proposition 8 rallied Wednesday in San Francisco. Credit... Jim Wilson/The New York Times

And for all the passion surrounding the issue of same-sex marriage, the question before the court is one that may seem technical, even dry: Does the initiative approved by Californians merely amend the State Constitution or, as gay rights groups hope the court will rule, revise it?

Under California law, an amendment is a matter that the state’s longstanding initiative process deals with routinely. A revision, however, entails a fundamental change to the Constitution, and requires approval of either two-thirds of each house in the Legislature or a constitutional convention. That could be much harder to achieve than passage in a referendum.

What elevates the ban on same-sex marriage to the level of a fundamental rewriting of the Constitution, opponents of Proposition 8 argue, is that it denies a right  the ability to marry  that the California Supreme Court earlier last year called inalienable. To take away that right now, they argue, would violate federal and state constitutional guarantees of equal treatment.

That court decision, in May, identified gay men and lesbians as a group that had historically suffered discrimination, and opened the door for some 18,000 same-sex couples to marry before Proposition 8 passed in November. The justices are also expected to rule on the validity of those marriages when they now decide the fate of the proposition.

Kenneth W. Starr, dean of the Pepperdine University School of Law and a former federal appeals judge and United States solicitor general, will argue before the justices on behalf of the measure’s backers. In a brief, Mr. Starr said efforts to overturn Proposition 8 ignored “the will of the people” expressed in an “open, fair election.”

But Shannon Minter, legal director of the National Center for Lesbian Rights, said that if a measure limiting what he described as the fundamental rights of gay people could be adopted by voter-approved amendment, then “any right can be taken away from any group” through a ballot measure. Mr. Minter will be Mr. Starr’s opponent at the hearing.