What is the test for a defendant to bring a claim that counsel's assistance was defective as to overturn a conviction?

" 'A convicted defendant's claim that counsel's assistance was so defective as to require reversal of a conviction . . . has two components.' [Citations.] 'First, the defendant must show that counsel's performance was deficient.' [Citations.] Specifically, he must establish that 'counsel's representation fell below an objective standard of reasonableness . . . under prevailing professional norms.' " (People v. Ledesma (1987) 43 Cal.3d 171, 216.) Second, he "must . . . establish prejudice." (Id. at p. 217.)