“Southern Dems”

[The Mikado]

Whenever you have a one party region, you will (almost by definition) most likely encompass a wide range of ideologies in one party. 

This. The entire political spectrum in the South was within the Democratic Party. By the 1930s you had New Dealer liberals in Congress from the South, and hidebound reactionary opponents to Roosevelt in Congress from the South. They just battled it out in the Democratic primary because there effectively wasn't a GOP to speak of.

[The Mikado]

NCYankee is right. It's also worth pointing out the effects of having a GOP that ranged from "underdog" (TN) to "gadfly" (most of Upper South) to "near-nonexistent" (Deep South) has. From the 1890s onward (barring the Smith election), if you wanted a say, you had to vote in the Dem primary, period. The total joke status of the GOP in the South surpasses the safest of safe jurisdictions in today's US (barring the poor DC GOP).