What is the test for deficient performance by a lawyer in a civil case?

Deficient performance by the attorney is not enough, however, to establish a Sixth Amendment violation. The petitioner must also establish that he was prejudiced by the attorney's deficient performance. The prejudice standard is "a reasonable probability that, but for counsel's unprofessional errors, the result of the proceedings might have been different." ¹⁷ Wong v. Belmontes¹⁸ holds that we cannot consider merely the best evidence the defense might have presented, but instead must consider all of the evidence that would have come into a penalty phase of a trial (both mitigating and aggravating) if the lawyer had made a different choice.¹⁹