SACRAMENTO, Calif. — Same-sex marriage opponents can’t keep the identities of their campaign donors secret, a federal appeals court ruled Tuesday in upholding a lower court decision.

A three-judge panel of the 9th U.S. Circuit Court of Appeals ruled against ProtectMarriage.com, the National Organization for Marriage and other supporters of Proposition 8, in part because the donors’ names have been public for years. The 2008 ballot initiative outlawed same-sex marriages in California until the ban was overturned last year.

The groups were fighting to remove the names from public view as they sought to conceal their past and future campaign finance records because they feared harassment of donors.

“The court’s ruling today recognizes the importance of campaign disclosure and turns back the attempted infringement on the public’s right to know,” Erin V. Peth, executive director of the California Fair Political Practices Commission, said in a statement.

But the court pointed to donor identities having been publicly available for five years now.

“In light of the disclosures, and their vast dissemination, we can no longer provide Appellants with effective relief,” Judge Milan D. Smith Jr. wrote for the majority, acknowledging that the court was in a “mootness Catch-22” because the information is already public. Judge J. Clifford Wallace partly dissented.

An attorney for the groups said that aspect of the ruling was encouraging.

“They did not reject the claim on the merits. They basically said the horse is already out of the barn, so the case is moot,” said John C. Eastman, an attorney for National Organization for Marriage who had helped argue the appeal.

He interpreted the ruling to encourage the groups to seek preliminary injunctions blocking the disclosure of donors’ names in future campaigns in other states.

“This issue arises all across the country, and I think the invitat ion from the court to seek a preliminary injunction will be very helpful,” Eastman said. “The concerns about harassment are very much alive, and we’re going to continue to protect our donors wherever these concerns arise.”

He could not immediately say if the groups would appeal.

The National Organization for Marriage solicited donations that were used to qualify the initiative, while ProtectMarriage.com is a coalition of religious conservative groups that sponsored and campaigned for the state constitutional amendment. Both said their donors were targeted for boycotts, hate mail and threats after their names appeared on the secretary of state’s website.

A lower court judge had refused to grant an emergency injunction that would have temporarily removed their names from public view.

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