Darling v. State
In Darling v. State, 520 P.2d 793, 794 (Alaska 1974) the supreme court explained the reasoning behind this rule:
"The vice sought to be guarded against by such a rule is the introduction into evidence of the unsworn testimony of counsel in which he states either explicitly or implicitly that his opinion as to the credibility of a witness is based upon personal knowledge of the witness."
At issue in Darling were remarks the prosecutor made during his closing argument to the jury, which were part of the formal record of the trial. (Id. at 793-94.)