Justice Marshall rejected both central elements of the Court's test. In his view, an undifferentiated objective-reasonableness standard gives too little practical guidance and improperly discourages lower courts from developing more concrete constitutional standards for such matters as investigation, client consultation, trial preparation, objections, and appeals.
He also disagreed with requiring a defendant to prove prejudice after establishing constitutionally inadequate lawyering. It is often impossible to reconstruct what competent counsel could have uncovered or achieved, especially when counsel's failures caused the record to be incomplete. More fundamentally, Marshall believed the right to effective counsel protects the fairness and integrity of the adversarial process, not merely the probability that the outcome would have changed.
At minimum, Marshall would have applied the standard more stringently in capital sentencing. Death is irrevocable, and effective mitigation investigation is indispensable to a reliable individualized sentencing decision. Washington's lawyer conducted virtually no meaningful investigation into available character and background evidence, despite material showing that relatives, friends, and others could have humanized Washington and described his family, work, church, and nonviolent traits.
Marshall concluded that counsel's failure to investigate could not be justified as strategy because counsel never adequately learned what mitigation evidence was available. Given the possibility that this evidence could have affected the sentencer's exercise of discretion, he would have vacated the death sentence and ordered a new sentencing proceeding. He also continued to believe that the death penalty is unconstitutional in all circumstances.