The Supreme Court waded into the fight over same-sex marriage at a time when public opinion is shifting rapidly in favor of permitting gay and lesbian couples to wed, but 40 states don't allow it.
The court's first major examination of gay rights in 10 years continues Wednesday, when the justices will consider the federal law that prevents legally married gay couples from receiving a range of benefits afforded straight married Americans.
The courtroom was packed on Tuesday and the crowd included actor-director Rob Reiner, who helped lead the fight against California's Proposition 8. Some people waited since Thursday — even through light snow — for coveted seats for the argument.
Both sides of the case were represented outside the courthouse. Supporters of gay marriage came with homemade signs including ones that read "a more perfect union" and "love is love."
Among the opponents was retired metal worker Mike Krzywonos, 57, of Pawtucket, R.I. He wore a button that read "marriage 1 man + 1 woman" and said his group represents the "silent majority."
Same-sex marriage is legal in nine states and the District of Columbia. The states are Connecticut, Iowa, Maine, Maryland, Massachusetts, New Hampshire, New York, Vermont and Washington.
Thirty states ban same-sex marriage in their state constitutions, while ten states bar them under state laws. New Mexico law is silent on the issue.
Polls have shown increasing support in the country for gay marriage. According to a Pew Research Center poll conducted in mid-March, 49 percent of Americans now favor allowing gays and lesbians to marry legally, with 44 percent opposed.
The California case was argued 10 years to the day after the court took up a challenge to Texas' anti-sodomy statute. That case ended with a forceful ruling prohibiting states from criminalizing sexual relations between consenting adults.
Kennedy was the author of the decision in Lawrence v. Texas in 2003, and he is being closely watched for how he might vote on the California ban. He cautioned in the Lawrence case that it had nothing to do with gay marriage, but dissenting Justice Antonin Scalia predicted the decision would lead to the invalidation of state laws against same-sex marriage.
Kennedy's decision is widely cited in the briefs in support of same-sex unions.
The California couples, Kris Perry and Sandy Stier of Berkeley and Paul Katami and Jeff Zarrillo of Burbank, filed their federal lawsuit in May 2009 to overturn the same-sex marriage ban that voters approved the previous November. The ballot measure halted same-sex unions in California, which began in June 2008 after a ruling from the California Supreme Court.
Roughly 18,000 couples were wed in the nearly five months that same-sex marriage was legal and those marriages remain valid in California.
The high-profile case has brought together onetime Supreme Court opponents. Olson, a Republican, and Democrat David Boies are leading the legal team representing the same-sex couples. They argued against each other in the Bush v. Gore case that settled the disputed 2000 presidential election in favor of George W. Bush.
On the other side Tuesday was Cooper, Olson's onetime colleague at the Justice Department in the Reagan administration.
The case is Hollingsworth v. Perry, 12-144.
Associated Press writer Jessica Gresko contributed to this report.