According to the Reference Dictionary, constituency refers to the body of voters or people who live in a particular voting district or the voting district served by an elected official. It also means any clientele of a particular base.
An elected official is responsible primarily to his constituency, as it consists of the people who elect him to represent them. For example, the constituency of a U.S. Senator from New York is comprised of the people of the state of New York, and that Senator is primarily responsible for representing their best interests. The word constituency is a derived from the word constituent, which means one who authorizes someone to represent him or act in his interest.