A rule aimed at curbing discriminatory behavior by lawyers is inciting heated debate in the legal industry, echoing pushback against antidiscrimination legislation playing out in the rest of the country.

The proposal, drafted by an American Bar Association committee, would deem it “professional misconduct” for a lawyer to harass or discriminate based on factors such as race, sex, religion or socioeconomic status. Under the model rule, engaging in such conduct, if “related to the practice of law,” would carry the risk of disciplinary...