In the case of Truesdale v. Aiken, Warden, et al., 1986, petitioner Robert Wayne Truesdale was convicted for murder and sentenced to death in South Carolina state court. He appealed his conviction on the grounds that he had been denied effective assistance of counsel during sentencing because his attorney failed to present mitigating evidence about his background and character. The Supreme Court held that a defendant must show not only that their lawyer's performance was deficient but also that this deficiency prejudiced their defense - meaning it affected the outcome of trial or sentencing. In this case, even though there were deficiencies in representation at sentencing phase (counsel did not investigate or present any mitigating evidence), no prejudice resulted from these deficiencies as there was overwhelming aggravating evidence against him including prior convictions for violent crimes and escape attempts while incarcerated.

In the dissenting opinion for Truesdale v. Aiken, Warden et al., Justice Brennan disagreed with the majority's decision to deny a stay of execution and dismiss the writ of certiorari as improvidently granted. He argued that there was substantial evidence suggesting that Truesdale had not received effective assistance from his counsel during sentencing, which could have resulted in an unjust death sentence. The court-appointed attorney did not present any mitigating evidence or even make a closing argument at sentencing; he also failed to object when the prosecutor made prejudicial remarks about Truesdale's silence after arrest. According to Brennan, these failures constituted ineffective assistance under Strickland v Washington standard and warranted further review by Supreme Court before carrying out capital punishment.