The man who has confessed to killing a high-profile Kansas abortion provider is expected to testify Thursday about his vehement opposition to abortion and what drove him to put a .22-caliber handgun to Dr. George Tiller's forehead and pull the trigger.

Scott Roeder is likely to repeat for jurors his already public claim that he believed killing Tiller, one of only a few late-term abortion providers in the U.S., was necessary to save the lives of unborn children.

His attorneys are expected to seek a conviction for voluntary manslaughter, rather than first-degree murder, to which Roeder has pleaded not guilty. Voluntary manslaughter in Kansas is defined as "an unreasonable but honest belief that circumstances existed that justified deadly force." A conviction on the lesser charge involves considerably less prison time.

District Judge Warren Wilbert said in court Wednesday that allowing a voluntary manslaughter charge also would mean proving an imminent danger of unlawful force. He noted abortion is legal in Kansas.

Wilbert will rule later on whether to allow jurors to consider the lesser charge.

Roeder, 51, of Kansas City, Mo., has also pleaded not guilty to two counts of aggravated assault for allegedly using a gun to threaten two ushers who tried to stop him after the May 31 shooting at Tiller's Wichita church.

While Roeder would be able to testify Thursday about his personal beliefs on abortion, Wilbert said he would not allow discussions about medical procedures or late-term abortions.

"Scott Roeder can testify to his beliefs, and the court is prepared to give him some pretty wide latitude," Wilbert said. But "we are not going to make this a referendum on abortion."

Defense attorney Mark Rudy said the defense burden will be to show Roeder believed his "actions were necessary."

The judge also will decide Thursday whether to allow former Kansas Attorney General Phill Kline, an anti-abortion Republican, 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.

But prospects for Kline's testimony appeared dimmer after the judge on Wednesday threw out a subpoena for Kansas Deputy Attorney General Barry Disney. Disney unsuccessfully prosecuted Tiller two months before he was killed on charges alleging Tiller failed follow Kansas requirements regarding late-term abortions.

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

Testimony in Roeder's trial, which has been under way since last week, has so far been comprised of prosecution witnesses who have described Tiller's shooting at the church he and his family attended, Roeder's purchase of a handgun days before the shooting, and Roeder's arrest hours after the shooting.