Can the suppression of material evidence helpful to the defense ever be harmless error, not least when the exclusion violated the Sixth Amendment's right to present a defense?

QUESTION PRESENTED On direct appeal, the state court found that constitutional error marred the trial, which ended with a death sentence for petitioner. The error — barring a defense expert from testifying that petitioner was incapable of forming the required mens rea—violated the Sixth Amendment's guarantee of the right to present a defense. The suppressed evidence was both “helpful” to the defense and “material,” said the court, adding, “if believed by the jury, the evidence certainly might have reduced the degree of homicide for which [petitioner] was convicted” to an offense ineligible for capital punishment. Still, the state court ruled the constitutional error was harmless, a conclusion affirmed on habeas review. The question presented is: Can the suppression of material evidence helpful to the defense ever be harmless error, not least when the exclusion violated the Sixth Amendment's right to present a defense? ii