As there was no indication that pursuing expert mitigation testimony in the penalty phase would cause conflicting narratives or a relitigation of the guilt phase, omitting to investigate the potential mitigation produced ineffective assistance of counsel; state appellate review to the contrary was unreasonable.
Where the state courts explicitly do not reach consideration of the prejudice arising from the ineffective assistance, federal habeas approaches the question de novo.
District court erred in determining that the mitigation would have been cumulative in light of the guilt-phase evidence; as the potential mitigation was qualitatively different, it is likely that at least one juror would have been swayed by a fundamentally different presentation in the penalty phase.
http://www2.ca3.uscourts.gov/opinarch/149001p.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.