Can a prosecutor deliberately vouch for the credibility of a key prosecution witness?

Where the determination of a defendant's guilt or innocence hinges almost entirely on the credibility of a key prosecution witness, allowing a conviction to be obtained by a prosecutor's deliberately vouching for that witness on behalf of the court would pose a clear threat to the integrity of judicial proceedings. That particular form of vouching goes beyond the mere proffer of an institutional warranty of truthfulness; rather, it casts the court as an active, albeit silent, partner in the prosecutorial enterprise. In doing so, it strikes at two principles that lie at the core of our system of criminal justice. The first of these is that "[t]he principle that there is a presumption of innocence in favor of the accused is the undoubted law, axiomatic and elementary...." Coffin v. United States, 156 U.S. 432, 453, 15 S.Ct. 394, 402, 39 L.Ed. 481 (1895). The second, long elevated to constitutional significance because it is so closely intertwined with the first, is that "to perform its high function in the best way 'justice must satisfy the appearance of justice.' " In re Murchison, 349 U.S. 133, 136, 75 S.Ct. 623, 625, 99 L.Ed. 942 (1955) (quoting Offutt v. United States, 348 U.S. 11, 14, 75 S.Ct. 11, 13, 99 L.Ed. 11 (1954)). If the prosecution may invoke the court as the guarantor of its truthfulness when the veracity of its star witness is challenged and can then survive review for plain error, both the actual likelihood and the perception that an accused will receive a fair trial--a trial in which he is presumed innocent and in which the government must prove every element of the charge against him beyond a reasonable doubt--are severely diminished. That result is untenable. Smith's conviction is accordingly reversed for plain error. ¹⁰