Twenty-two pages into the hand-scribbled journal found in Dylann S. Roof’s car — after the assertions of black inferiority, the lamentations over white powerlessness, the longing for a race war — comes an incongruous declaration.

“I want state that I am morally opposed to psychology,” wrote the young white supremacist who would murder nine black worshipers at Emanuel A.M.E. Church in Charleston, S.C., in June 2015. “It is a Jewish invention, and does nothing but invent diseases and tell people they have problems when they dont.”

Mr. Roof, who plans to represent himself when the penalty phase of his federal capital trial begins on Tuesday, apparently is devoted enough to that proposition (or delusion, as some maintain) to stake his life on it. Although a defense based on his psychological capacity might be his best opportunity to avoid execution, he seems steadfastly committed to preventing any public examination of his mental state or background.

“I will not be calling mental health experts or presenting mental health evidence,” he wrote to Judge Richard M. Gergel of Federal District Court on Dec. 16, a day after a jury took only two hours to find him guilty of 33 counts, including hate crimes resulting in death, obstruction of religion and firearms violations. At a hearing on Wednesday, Mr. Roof told the judge that he planned to make an opening statement but not call witnesses or present evidence on his behalf.

After receiving the results of a psychiatric examination in November, Judge Gergel found Mr. Roof competent to stand trial — meaning that he was capable of understanding the proceedings and assisting in his defense. At Mr. Bruck’s request, the judge scheduled a second competency hearing for Monday, but he signaled last week that he saw no reason to delay the penalty phase.

The judge has repeatedly warned Mr. Roof against representing himself, including immediately after the verdicts, when he called it “a bad decision” and urged him to “fully appreciate the implications.” The warnings have had no discernible effect on Mr. Roof, who has until Tuesday to reverse his decision to relegate his lawyers to standby counsel. That status allows Mr. Bruck and his team to offer guidance, but not to question witnesses or make objections.

Prosecutors plan a procession of grief, perhaps calling dozens of members of victims’ families to testify about the impact of the killings. The prosecutors are also likely to re-emphasize the considerable evidence of Mr. Roof’s premeditation and clearly articulated racial intent. Death penalty experts said the absence of mental health evidence to mitigate those aggravating factors could be decisive.

“If the jury views Roof as evil and having made a knowing, intelligent choice to kill these innocent, churchgoing people in order to foment racial hatred, they are much more likely to impose the death penalty than if they believe him to be a young and severely mentally ill person who acted under delusional racist beliefs,” said Robert Dunham, executive director of the Death Penalty Information Center, a research group.

It would take only one holdout on the jury, which consists of 10 women and two men, to spare Mr. Roof from lethal injection. Judge Gergel has ruled that the jurors can be told that prosecutors had rejected Mr. Roof’s offer, through Mr. Bruck, to plead guilty in exchange for a life sentence.

Many in Charleston were relieved by Mr. Roof’s conviction in light of the mistrial that had been declared 10 days earlier in the state murder prosecution of Michael T. Slager, the white North Charleston policeman whose fatal shooting of a black motorist in 2015 was captured on video.

Even those who oppose the death penalty on moral grounds, like the Rev. Joseph A. Darby, a presiding elder for the African Methodist Episcopal Church in Charleston, said it would seem bewildering for Mr. Roof to escape capital punishment.

“That could very well be the end of the death penalty in America, because if there was ever justification for killing anybody, this is the case,” Mr. Darby said.

There is no consensus among members of the victims’ families about Mr. Roof’s fate. When the Justice Department elected in May to seek the death penalty, it acted against the wishes of many and of the two women he had spared. Five relatives offered Mr. Roof a measure of forgiveness at a remarkable bond hearing two days after the shootings. But by law, those who testify now are prohibited from telling the jury what penalty they think he should receive.

“It’s going to be extremely emotional, powerful testimony,” said John H. Blume, a death penalty expert who teaches at Cornell Law School, “and that emotion could implicitly and misleadingly indicate to the jury that some of these people want the death penalty when it’s not the case.”

If Mr. Roof is sentenced to death, it will be the first time a jury has done so in a prosecution involving the federal hate crimes law, according to experts on capital cases. That statute, which was broadened in 2009, does not carry a potential death sentence, but Mr. Roof was also convicted of other crimes that do.

A death sentence most likely would give way to a yearslong series of appeals (in which Mr. Roof could not represent himself). Among the issues could be the composition of the jury, given that Mr. Roof acted rather passively as his own lawyer when it was selected; the withholding of evidence on mental health and other mitigating factors; and Mr. Roof’s competence to stand trial and to represent himself.

In 2008, the Supreme Court ruled in Indiana v. Edwards that trial judges could insist on legal representation for defendants who are “competent enough to stand trial but who suffer from severe mental illness to the point where they are not competent to conduct trial proceedings by themselves.”

Mr. Bruck and his team have argued in court filings that Mr. Roof, a ninth-grade dropout, “has no right to represent himself in a capital trial, and even less so at the penalty phase.” But in the 41 years since the Supreme Court recognized a Sixth Amendment right of self-representation for criminal defendants, in Faretta v. California, the court has never specifically narrowed that holding for death penalty trials, despite their complexity.

Some death penalty opponents hope that Mr. Roof’s defiance will prompt the appellate courts to adopt a more rigorous standard for capital defendants.

“Whether or not they’re legally insane, there’s certainly something mentally wrong with them,” said Peter D. Greenspun, a lawyer who was ousted by a defendant, John A. Muhammad, for part of a capital murder trial for the 2002 sniper attacks in the Washington area.

He added, “To have a person like that make this kind of decision, it really calls into question, from a philosophical point of view, whether that person is in a position to understand their civil liberties.”

Mr. Muhammad, who ultimately reinstated Mr. Greenspun, was sentenced to death in 2003 and executed six years later.

“It’s something that Roof will likely regret,” Mr. Greenspun predicted of his choice to represent himself. “At some point down the road, he’s going to say, ‘What did I do?’ And there’s no going back.”