Wichita, Kansas (CNN/In Session) -- Jury selection begins this week in the murder trial of a man who admits he fatally shot Dr. George Tiller, one of four abortion providers in the country who performed late-term abortions.

But if defendant Scott Roeder has his way, abortion also will be on trial when testimony begins.

Roeder, 51, has never denied shooting Tiller. In statements to the news media and in pretrial memos, he has asserted that he was justified under the theory that he was trying to save the lives of unborn children when he gunned down Tiller at Sunday church services on May 31, 2009.

Tiller, who had been performing late-term abortions in his Wichita clinic since 1973, was serving as an usher at Reformation Lutheran Church when Roeder, by his own admission, shot him in the head.

Read the criminal complaint

Sedgwick County prosecutors are fighting to keep the issue of abortion out of the trial, and even the presiding judge has said he does not want the case to become a venue for a debate on the legality of abortion.

But Judge Warren Wilbert has not made a final ruling on what role, if any, the abortion issue will play in the trial.

Jury selection was delayed Monday when prosecutors filed additional motions asking the judge to keep tight reins on Roeder's planned defense.

"The state urges this court to exclude any irrelevant evidence of abortion, the defendant's views on abortion, and the character of Dr. Tiller," prosecutors wrote in their court papers.

With Roeder's beliefs expected to be the focal point of the defense case, the trial could become the next forum in the fierce debate over one of the nation's most emotionally charged and divisive issues.

But prosecutors said in their court papers that the defense theory is legally flawed.

"Case law is clear, both in Kansas and nationwide, that the circumstances of this murder do not justify an imperfect self-defense instruction," they argued in their court papers.

Security is expected to be tighter than usual at the courthouse when proceedings begin Wednesday with jury selection, and officials are uncertain whether demonstrators will gather for the trial. Some groups, including Wichita-based Operation Rescue, have distanced themselves from violence as a means of protest.

Throughout his controversial career, Tiller was among the most enduring targets of the anti-abortion movement, withstanding several legal challenges and numerous death threats, including one in 1993 in which he was shot in the arms. His death sent shock waves through both factions in the abortion debate, with most from the anti-abortion movement rushing to condemn Roeder's actions.

Roeder faces life in prison if convicted of first-degree murder in Tiller's death. He also is charged with two counts of aggravated assault for allegedly pointing his gun at two other ushers as he fled the church.

In several media interviews after his arrest, Roeder expressed no remorse over shooting Tiller, claiming it was necessary to prevent the doctor from carrying out controversial late-term abortions at the Women's Health Care Services clinic.

"Defending innocent life, that is what prompted me," he told The Associated Press in a November 10 telephone interview.

He also spoke of his intention to raise a so-called "necessity defense," a stance that put him at odds with his public defenders, who told the media that such a defense did not exist under Kansas law.

Judge Warren Wilbert has twice denied Roeder's request on the grounds that it is not applicable under Kansas law, which considers abortions legal. The most recent request came on Friday.

Wilbert warned Friday that he did not want his courtroom to become the setting for a full-blown debate on abortion. He did, however, indicate that he would consider giving jurors the option of convicting Roeder of voluntary manslaughter, which is defined as "an unreasonable but honest belief that circumstances existed that justified deadly force" under Kansas statute.

His comments opened the door for Roeder's lawyers to try to present evidence to support his belief that he had no other option than to kill the doctor.

It is a door prosecutors want to keep closed. "Taken to its logical extreme, this line of thinking would allow anyone to commit premeditated murder, but only be guilty of manslaughter, simply because the victim holds a different set of moral and political beliefs than the attacker, prosecutors wrote in court papers filed Monday.

The prosecutor's latest legal motions were prompted by the judge's comments from the bench on Friday.

"This will not become a trial of the abortion issue, and there are not going to be witnesses who will testify [to] graphic descriptions of abortion procedures and revisit and argue all the legally insufficient discussions and debates over the harm caused," Wilbert said. "It will be limited to Mr. Roeder's beliefs, and how he came to form those beliefs."

Wilbert added that he would remain "open-minded" to a self-defense theory.

The judge's remarks surprised observers on both sides of the abortion debate, while raising the possibility of significantly less prison time. Sentencing guidelines for voluntary manslaughter range from 55 months to 247 months, depending on the crime and the defendant's criminal history.

"(The) perplexing decision is effectively backdoor permission for admitted killer Scott Roeder to use a 'justifiable homicide' defense that is both unjustifiable and unconscionable," said Katherine Spillar, executive vice president of Feminist Majority Foundation.

"Allowing an argument that this cold blooded, premeditated murder could be voluntary manslaughter will embolden anti-abortion extremists and could result in 'open season' on doctors across the country," Spillar added.

Prosecutors contend the so-called "imperfect defense" theory doesn't apply to Roeder's case because of evidence the crime was planned in a logical fashion.

"Kansas courts have consistently held that an imperfect self-defense instruction is improper in situations where there is substantial evidence of premeditation, such as choosing in advance the time and manner of the killing," the court papers stated.

"Because evidence of premeditation shows reasoning, premeditation and imperfect self-defense are mutually exclusive theories," prosecutors concluded.

One of Roeder's supporters expressed disappointment that the judge had rejected the necessity defense, but remained hopeful that the issue of the legality of abortion would have its day in court.

"I am grateful for this evidence that the judge's heart is not fully closed," said David Leach, an Iowa-based anti-abortion activist and friend of Roeder's, who wrote his pro se briefs on the necessity defense. "I think pro-deathers are right to worry that death may lose."