American Bar Association Model Rules of Professional Conduct - Rule 6.2. Accepting Appointments
A lawyer shall not seek to avoid appointment by a tribunal to represent a person except for good cause, such as:
(a) representing the client is likely to result in violation of the Rules of Professional Conduct or other law;
(b) representing the client is likely to result in an unreasonable financial burden on the lawyer; or
(c) the client or the cause is so repugnant to the lawyer as to be likely to impair the client-lawyer relationship or the lawyer's ability to represent the client.