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.