Laurence Tribe, a constitutional law professor at Harvard Law School, said the court had "considerable sympathy for the idea of letting the issue percolate further at the state level rather than imposing a one size-fits all federal solution."
Several justices suggested that the proposition's sponsors lacked legal "standing" to appeal after a federal judge declared the measure unconstitutional in 2010. The standing issue is important because California state officials declined to defend Proposition 8 in court.
A Supreme Court decision based on lack of standing would reinstate a federal trial judge's ruling that struck down Proposition 8. The two sides would then be left to fight over how far the judge's ruling extends. Proposition 8 supporters say it would apply at most to the two gay couples challenging the law.
Opponents say the impact would be much broader, legalizing gay marriage in California, the most populous state. Its 38 million residents are 12.1 percent of the U.S. population of 314 million, according to a Census Bureau estimate in 2012.
"We would view that as a major win," said Theodore Boutrous Jr., one of the lawyers challenging Proposition 8, in a telephone interview.
Although "we would like to win in the broadest possible way" with a nationwide right to gay marriage, Boutrous said, "it would again fuel the momentum in this country that we think our case has been part of causing."
Public support for gay marriage is at record levels in opinion surveys. A Pew Research Center poll released last week found that 49 percent of adults supported legalization, with 44 percent opposed. Ten years earlier, a Pew poll found only 33 percent in support, with 58 percent opposed. Nine states and the District of Columbia now let gay couples marry.
The justices indicated they were more likely to issue a direct ruling on the Defense of Marriage Act, a 1996 law that defines marriage as being between a man and a woman and denies gay couples the benefits available to heterosexuals. The Obama administration is urging the court to strike down the law, known as DOMA.