Caesar's Legion was from the Midwest, they fled there after the NCR defeated them in a surprise attack, during a saturnalia style orgy during the First Battle of Hoover Dam. There were also radio-active twisters present all across the midwest, which prevents makes travel for certain seasons improbable. This was all before the Bethesda times, however.

Chicago has an enclave outpost likely, there is a brotherhood chapter in montana and outside of chicago. More besides the game-that-shall-not-be-named we do not know about the midwest.