An increase in misdemeanor prosecutions over the past several decades and a lack of clarity over prosecutor responsibilities in part prompted the American Bar Association to issue an opinion on prosecutor obligations when negotiating pleas.

The “primary duty of a lawyer engaged in public prosecution is not to convict, but to see that justice is done,” the opinion from the ABA’s Standing Committee on Ethics and Professional Responsibility said.

But since 1972, the number of misdemeanor prosecutions “is estimated to...