Question

People in districts represented in Congress are called

Answer 1

The answer is: constituents.

A constituent is a resident of a place that is represented by a politician who has been elected to represent the interests of the residents of that place in question (a district in this case).

When a constituent exercises his or her power of choice through an election, by casting a ballot for his or her choice, he or she then becomes known as a voter.  

Answer 2

People in a congressional district are called constituents.