EUGENE, Ore. — A federal judge ruled Wednesday that a national group cannot defend Oregon’s ban on same-sex marriage after the state attorney general refused to do so, paving the way for a ruling on the ban that could come at any time.

The National Organization for Marriage argued the group should be allowed to intervene on behalf of its members in Oregon.

Judge Michael McShane said the attorney general is accountable to Oregon voters, while the group supporting marriage solely between a man and woman is not.

“I’m not prepared to substitute a third party for the executive branch,” McShane said.

He found the organization did not file its request in a timely manner and failed to show that its members had specific interests that needed to be protected.

The group’s chairman, John Eastman, said he plans to appeal the ruling on standing to the 9th U.S. Circuit Court of Appeals.

“These things never end at the trial court,” Eastman said.

The organization declined to identify its Oregon members, citing fears they will be harassed, but said they include a county clerk who issues wedding licenses; the owner of a wedding business; and a voter who cast a vote in favor of the 2004 ballot measure that added a same-sex marriage ban to the state Constitution.

The group’s move was opposed by lawyers for the state and for four gay and lesbian couples, who brought two cases that later were merged.

Attorney General Ellen Rosenblum, a Democrat, said the ban is legally indefensible and had urged the judge to throw it out, creating a rare case where both plaintiffs and defendants sought the same outcome.

The judge has not said when he plans to rule on the underlying case.

“It’s real for us,” said Ben West, one of the plaintiffs who brought the case. “There’s talking heads and politics, there’s controversy and adversary, there’s all that drama around it. But we’re real people and these are our families, and it really impacts us.”

Advocacy groups say they have collected enough signatures to ask voters in November whether same-sex marriage should be legal. They have said they would discard the signatures if the judge rules in their favor by May 23. But the prospect of an appeal by the National Organization for Marriage could change things.

Jeana Frazzini, director of Basic Rights Oregon, was less firm about that deadline. She said the gay-rights group will “thoughtfully assess the landscape as it continues to change.” The group has until July 3 to submit just over 116,000 valid signatures.

Last year, the U.S. Supreme Court ruled that proponents of California’s same-sex marriage ban, known as Proposition 8, did not have legal authority to appeal a lower court ruling that overturned the initiative.

Oregon law has long prohibited same-sex marriage, and voters added the ban to the state constitution in 2004.

The decision, approved by 57 percent of voters, came months after Multnomah County, which is the state’s largest and includes Portland, briefly issued marriage licenses to same-sex couples. About 3,000 gay couples were allowed to marry before a judge halted the practice. The Oregon Supreme Court later invalidated the marriages.

In filings with the court and statements to the media, Eastman had said that he was concerned the judge may be required to recuse himself because, as a gay man raising a child, he shares characteristics with some of the plaintiffs.

McShane addressed the issue in court before handing down his ruling, saying he’s never attended a rally or spoken publicly about gay marriage, and he can’t recall ever donating money to gay-rights causes.

McShane said gay men have regularly appeared before him in criminal, civil and family cases, sometimes with their children.

“I’ve sent people with very similar characteristics to me to prison,” McShane said. “To me, it’s irrelevant.”