"The object of California's establishment of the legal relationship of domestic partnership is to grant committed same-sex couples rights equivalent to those accorded a married couple. but Proposition 8, by depriving same-sex couples of the right to marry, denies them the dignity, respect, and stature accorded similarly situated opposite-sex couples under state law," the court brief said.
Such civil union laws in most cases provide the same rights of marriage under state law, without actually calling it that.
Dozens of advocacy groups on both sides of the issue have bombarded the high court with briefs, including a coalition of national Republicans, business, faith, and military leaders supporting same-sex marriage.
Among the prominent conservative names lending their view: former Utah governor and presidential candidate Jon Huntsman, Hewlitt-Packard chief executive and former California gubernatorial candidate Meg Whitman, U.S. Rep. Ileana Ros-Lehtinen (R-Florida), and actor Clint Eastwood.
"As a Republican, I believe in protecting individual freedoms and that everyone, including gay and lesbian Americans, has a constitutional right to be treated equally under the law," said former Rep. Jim Kolbe.
California state officials, including Gov. Jerry Brown, a Democrat, also weighed in to oppose Proposition 8.
"California's interests in protecting all of its children are best served by allowing these same-sex couples to enjoy the same benefits of marriage as opposite-sex couples," state Attorney General Kamala Harris said of the estimated 50,000 youngsters being raised by gay and lesbian couples in the state.
Obama has had an evolving position on gay rights, once supporting only civil unions. But in his inaugural address last month, he raised expectations, and perhaps signaled his impending legal views, when offering sweeping rhetoric.
"Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law-- for if we are truly created equal, then surely the love we commit to one another must be equal as well."
In February, a federal appeals court in San Francisco ruled the California measure unconstitutional. In its split decision, the panel found that Proposition 8 "works a meaningful harm to gays and lesbians" by denying their right to civil marriage.
The Supreme Court has discretion to rule narrowly or broadly on the aspects of the legal and procedural questions raised.