The National Organization for Marriage (NOM) said it would file a motion in federal court on Monday seeking to intervene in a lawsuit challenging Oregon's ban on gay marriage.

Arguments in the case are set for Wednesday with U.S. District Judge Michael McShane presiding.

Oregon voters in 2004 approved Measure 36, a constitutional amendment which limits marriage to heterosexual couples.

As of Sunday, no legal argument for upholding the ban had been submitted. And Attorney General Ellen Rosenblum not only called the ban legally indefensible, she encouraged McShane to throw it out, saying that the state is prepared to begin issuing marriage licenses to gay and lesbian couples.

NOM President Brian Brown accused Rosenblum of “abandoning her duty” and having “switched sides.”

“As a membership organization, we speak on behalf of our members, including a County Clerk in the state, several professionals in the wedding industry, and voters,” Brown said in a blog post. “All of these individuals have a particularized interest in the outcome of the litigation, yet their interests are not being represented. We are working to protect the interests of our members who support true marriage against a collusive lawsuit that has the state joining with the plaintiffs against the interests of our members, and the state's voters.”

NOM Chairman John Eastman added that McShane's sexual orientation – he's gay – raises “troubling questions about his impartiality.”

“If our motion to intervene is granted, we intend to fully and aggressively defend the state constitutional amendment,” Eastman said.