A constitutionalist is a person who believes that a government should be based on a constitution — that even leaders must obey those basic rules. The word also works as an adjective: a constitutionalist view is one that puts the constitution first. In history, “constitutionalists” are sometimes a named group, for example people who wanted a constitution instead of one-person rule. It is a formal, political word, more at home in newspapers and essays than in casual chat. Though actually, the ending is the part to hold onto: -ism (constitutionalism) is the belief; -ist (constitutionalist) is the person. You believe in constitutionalism, or you are a constitutionalist. You do not “believe in constitutionalist.”