What constitutes a "crucial" defense?

The exculpatory evidence which was not introduced at defendant's trial constitutes a "crucial" defense, as required by In re Saunders (1970) 2 Cal.3d 1033, 1049, 88 Cal.Rptr. 633, 472 P.2d 921), "the possible defense so withheld must be termed a 'crucial' one." A defendant is denied the effective assistance of counsel if, " 'by reason of counsel's failure to perform the obligations imposed upon him, defendant is deprived of an adjudication of a crucial or potentially meritorious defense.' " (People v. Shaw (1984) 35 Cal.3d 535, 541, 198 Cal.Rptr. 788, 674 P.2d 759, emphasis in original.)