The following excerpt is from USA. v. Sayakhom, 186 F.3d 928 (9th Cir. 1999):
The advocate-witness rule prohibits an attorney from appearing as both a witness and an advocate in the same litigation. See United States v. Prantil, 764 F.2d 548, 552-53 (9th Cir. 1985). The policies underlying this rule "are related to the concern that jurors will be unduly influenced by the prestige and prominence of the prosecutor's office and will base their credibility determinations on improper factors." United States v. Edwards, 154 F.3d 915, 921 (9th Cir. 1998).
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