Porter v. McCollum, 558 U.S. 30 (Nov. 30, 2009)

A capital defendant’s trial counsel’s conduct fell below an objective standard of reasonableness when counsel failed to investigate and present mitigating evidence, including evidence of the defendant’s mental health, family background, and military service. The state court’s holding that the defendant was not prejudiced by counsel’s deficient representation was unreasonable. To establish prejudice, the defendant need not show that counsel’s deficient conduct more likely than not altered the outcome; the defendant need only establish a probability sufficient to undermine the confidence in the outcome, as he did in this case.