Justice Kennedy often provides the decisive vote in ideologically divided decisions, and he has taken a special interest in cases concerning gay rights. He also wrote the majority opinion in Lawrence v. Texas, which struck down laws making gay sex a crime.

Therese Stewart, a lawyer for the City of San Francisco, which joined the lawsuit challenging Proposition 8, said there was a lot to like about Judge Reinhardt’s approach.

“We’ve got a victory,” Ms. Stewart said. “It not only lessens the likelihood that the court feels compelled to take the case, but it also gives the court an alternative route to rule in our favor.”

As for persuading the justices to find that there is an across-the-board right to same-sex marriage in the Constitution, she said, “It’s a fairly big ask.”

The losing side in the case must soon decide whether to seek review from a larger panel of the Ninth Circuit or head directly to the Supreme Court. Members of its legal team would not discuss their strategic calculations.

There have been suggestions, though, that they would be better off to seek further review in the appeals court. They could win, or, failing that, they could lose more thoroughly and so elicit a decision that the Supreme Court would feel compelled to hear and be more likely to reverse.

The winning lawyers, on the other hand, have long maintained that their ultimate destination is the Supreme Court, and it was not clear if they would be content with a victory limited to California. They said they would oppose a request for Supreme Court review, though perhaps not very strenuously.