[durgiston]

Actually, it is an act of Congress that mandates single-member geographical districts (1967 Single-Member District Mandate). Several states over the years have had at-large districts, and it has been a debate since at least the 1842 Apportionment Act. The debate has gone from one focused on federalism (states should decide how to elect reps) to equal protection (black people should get not get drowned out by racist southern states. flip side, gerrymandering is easy to do, hard to prevent). Nothing in the US Constitution prevents at-large House reps.