The right to counsel guaranteed by the Constitution contemplates the services of an attorney devoted solely to the interests of his client. . . . “The lawyer owes ‘entire devotion to the interest of the client, warm zeal in the maintenance and defense of his rights and the exertion of his utmost learning and ability,’ to the end that nothing be taken or be withheld from him, save by the rules of law, legally applied. No fear of judicial disfavor or public unpopularity should restrain him from the full discharge of his duty. In the judicial forum, the client is entitled to the benefit of any and every remedy and defense that is authorized by the law of the land, and he may expect his lawyer to assert every such remedy or defense.”
Von Moltke v. Gillies (1948) 332 U.S. 708, 725-726, fn. 9.
“. . . defense counsel has no comparable obligation to ascertain or present the truth. Our system assigns him a different mission. He must be and is interested in preventing the conviction of the innocent, but, absent a voluntary plea of guilty, we also insist that he defend his client whether he is innocent or guilty. The State has the obligation to present the evidence. Defense counsel need present nothing, even if he knows what the truth is. He need not furnish any witnesses to the police, or reveal any confidences of his client, or furnish any other information to help the prosecution’s case. If he can confuse a witness, even a truthful one, or make him appear at a disadvantage, unsure or indecisive, that will be his normal course. Our interest in not convicting the innocent permits counsel to put the State to its proof, to put the State’s case in the worst possible light, regardless of what he thinks or knows to be the truth. Undoubtedly there are some limits which defense counsel must observe but more often than not, defense counsel will cross-examine a prosecution witness, and impeach him if he can, even if he thinks the witness is telling the truth, just as he will attempt to destroy a witness who he thinks is lying. In this respect, as part of our modified adversary system and as part of the duty imposed on the most honorable defense counsel, we countenance or require conduct which in many instances has little, if any, relation to the search for truth.”
United States v. Wade (1967) 388 U.S. 218, 256-258.