This is basically how I understand the term. States in red are in the Midwest, states in green have portions in the Midwest but not the whole state. Only the eastern portions of the Dakotas, Nebraska and Kansas and only the portions of West Virginia and Kentucky along the Ohio River are in the Midwest. On the poll, I included the Western group of green states, but not the Eastern two.
This for me, except I'd put Missouri as green, too. Only St. Louis is truly culturally Midwestern, and you can even tell a greater Southern/Western influence there than you can in Illinois.
EDIT: WV is in no way, shape or form part of the Midwest, IMO.