Final answer:
A lawyer is prohibited from acting as both an advocate and a witness to maintain impartiality and ensure effective legal representation as dictated by the Sixth Amendment. Cases like Glasser v. United States and Strickland v. Washington highlight the importance of conflict-free and competent counsel.
Step-by-step explanation:
The prohibition against a lawyer acting as an advocate in a trial where they are likely to be a necessary witness is based on the principle of impartiality and the avoidance of conflicts of interest. This rule ensures that a lawyer's dual role does not compromise the right to an attorney as guaranteed by the Sixth Amendment.
The Assistance of Counsel Clause of the Sixth Amendment was significantly interpreted in Glasser v. United States, which emphasized the importance of conflict-free representation.
Additionally, the landmark case of Strickland v. Washington sets forth the standard for assessing ineffective assistance of counsel, thereby ensuring the accused's right to a competent legal defense.