I was looking again at the maps of which way voters swung from 2004 to 2008, and noticed an odd anomaly: a hard line at Arkansas' northern, southern, and eastern borders (and to a lesser extent at Tennessee and Oklahoma borders).
At Arkansas' eastern border, well, there's a big river there. But elsewhere, wouldn't you expect a more gradual fade?
Aside from vague and hardly-convincing psychological surmises (strong conservatives prefer living in northern Arkansas to southern Missouri?), the only explanations I can imagine for this are election-related. Is there something about registration and/or voting in Arkansas and Tennesee that causes this abrupt statistical shift at the borders?