American Bar Association Model Rules of Professional Conduct - Rule 3.1. Meritorious Claims and Contentions

Rule

A lawyer shall not bring or defend a proceeding, or assert or controvert an issue therein, unless there is a basis in law and fact for doing so that is not frivolous, which includes a good faith argument for an extension, modification or reversal of existing law. A lawyer for the defendant in a criminal proceeding, or the respondent in a proceeding that could result in incarceration, may nevertheless so defend the proceeding as to require that every element of the case be established.

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See American Bar Association Model Rules of Professional Conduct