Some conservative groups are now gearing up for a court fight over Illinois’ same-sex marriage ban, after the top lawyers for the state and for Cook County annouced they will not fight lawsuits challenging the constitutionality of Illinois marriage law.

Two conservative groups - the Thomas More Society and the Illinois Family Institute - say they will try to intervene in a lawsuit challenging the state’s gay marriage ban some time next week. The ban is being challenged by 25 same-sex couples who were turned away when they tried to get marriage licenses from the Cook County Clerk’s office.

But in a rare move, Illinois Attorney General Lisa Madigan and Cook County State’s Attorney Anita Alvarez have indicated they won’t defend the current state law. Both Democrats say they think the state’s gay marrige prohibition violates the Illinois constitution.

Thomas More Society Executive Director Peter Breen said his group will try to intervene in the case as a third party. He said he’s been reaching out to county clerks and state’s attorneys all over Illinois to help him mount a defense.

“You’re gonna have all of these couples from all over the state of Illinois get marriage licenses here in Cook County and then take them home, and then wreak havoc on their own counties’ mariage laws,” Breen said. “The state of Illinois can’t have two marriage laws.”

DuPage County Clerk Gary King, a Republican, confirmed he’d been contacted by the group about jumping into the gay marriage case.

“I don’t remember a court case so iomportant where all the parties are - no one is presenting both sides,” King said. “And I think that’s the issue. I’d hate to see the law changed this way.”

But King said he is still unsure whether he’ll try to intervene in the Cook County lawsuits.

Meanwhile, the head of the Illinois Family Institute said it will file court papers in the coming days.

“We have very sincere concerns about how this is going to affect religious liberty,” said Executive Director David E. Smith. He cited Illinois’ recent flap with Catholic Charities over same-sex civil unions, when the state cancelled its contract with the group because it refused to provide adoptions to couples who weren’t married.

Even if opponents of the same-sex marriage push in Illinois organize a defense of the current state law, a federal judge would still have to rule that they have standing to intervene in the Cook County case as third parties.

Lawyers for the American Civil Liberties Union of Illinois and Lambda Legal, which launched the challenges to Illinois’ law, said third party intervention would not change their arugment that the same-sex marrige ban is unconstitutional.