Defense attorneys have "a formidable and daunting task" ahead if the man who has confessed to killing a Kansas abortion provider is hoping for a chance at a lesser sentence by arguing he sincerely believed his actions were necessary to save unborn children, a judge said Wednesday.

Scott Roeder will take the witness stand to testify on his own behalf, attorney Steve Osburn told The Associated Press in the wake of a heated hearing about which defense evidence jurors will be allowed to hear.

Osburn said the defense expects to present its case in a single day Thursday.

Roeder, 51, is charged with premeditated, first-degree murder in the May 31 shooting of Dr. George Tiller, one of the nation's few late-term abortion providers. The Kansas City, Mo., man also was charged with two counts of aggravated assault for allegedly using a gun to threaten two ushers who tried to stop him after the shooting.

Attorney Mark Rudy confirmed to the court for the first time Wednesday that the defense would build a case based on the lesser charge of voluntary manslaughter — defined in Kansas as "an unreasonable but honest belief that circumstances existed that justified deadly force."

A voluntary manslaughter conviction for someone with little criminal history carries a sentence of about five years, compared to the life sentence Roeder faces if convicted of first-degree murder.

"I didn't enact the voluntary manslaughter statute — the Legislature did," District Judge Warren Wilbert said.

But Wilbert reminded attorneys they must couple a voluntary manslaughter defense with a showing of imminent danger posed by the doctor. Wilbert will rule at the end of the defense's case whether there is sufficient evidence to instruct jurors that they can consider the lesser charge.

Roeder will be allowed to testify about his personally held beliefs, the judge said, not about medical procedures of which he has no knowledge or expertise.

"He is not going to be able to get up there and just blurb out what he wants to say," Wilbert said.

Also Wednesday, Wilbert threw out a subpoena defense attorneys had issued to Kansas Deputy Attorney General Barry Disney, the lead prosecutor in a case that brought 19 misdemeanor alleging Tiller failed to obtain a second opinion for late-term abortions from an independent physician, as required by Kansas law. Tiller was acquitted just two months before his death after a trial Roeder has said he attended.

Rudy had argued Disney's testimony was a necessary "building block" in the defense case to show Roeder went to the trial and relied on Disney's honest belief as a prosecutor that Tiller was breaking the law.

"The state does not believe any of this mish mash is relevant ... to this case," District Attorney Nola Foulston countered. "This is the kind of psychotic, circuitous kind of logic we are dealing with."

The judge told the defense they could put their client on the stand to make the same point.

"Scott Roeder can testify 'til the cows come home about the (Tiller) trial," Wilbert said.

The judge also will decide Thursday whether to allow former Kansas Attorney General Phill Kline to testify. Kline, now a visiting assistant law professor at Liberty University in Lynchburg, Va., filed misdemeanor charges against Tiller in 2006 that were dismissed in a jurisdictional dispute with Foulston.

"We are not going to make this a referendum on abortion," Wilbert said.

The American Civil Liberties Union filed a friend-of-the-court brief Wednesday on behalf of the National Abortion Federation, the ACLU and the ACLU of Kansas asking the court to bar Roeder from arguing his anti-abortion beliefs in support a voluntary manslaughter conviction.

Vicki Saporta, president of the National Abortion Federation, said outside the courtroom that if Roeder is allowed to argue that his anti-abortion beliefs justify reducing his accountability "we fear for the safety of abortion providers" and women's access to abortions.

Eight abortion providers have been killed since 1977 and 17 others have faced murder attempts. That is in addition to 175 arsons and 41 bombings, Saporta said.