Very nice! I'm looking at the 1920 map and thinking "and people say we're too polarized now?"
Exit polls going back to 1972 show it's by far the most polarized it's been starting with 1972. Too bad we don't have exit polls from the '20s and '30s.
It is true that polarization has followed a rough parabolic shape from 1920 to today. Before 1928, the partisan bases were sharply defined and relatively homogenous. They could be determined through an economic basis derived from the agricultural economy, which in turn was derived soil fertility and ultimately from climate.
Northern white ethnics and blacks' invasion of the Democrats was then followed by southern Baptists' invasion of the Republicans that year. This threw the postwar partisan alignment into chaos. The agricultural basis was no longer relevant in the industrial society.
The chaos transition period (1928 onwards) is now coming to a close. The spoke-and-hub artificial transportation system has replaced the natural clime-based agricultural system as the basic unit of economic activity, and by derivation, of its contribution to the determination of politics.
The Republicans have found a new economically determined base around the spoke of the modern spoke-and-hub transportaiton system and the Democrats have found a similiar base near the hub of the system. The demise of Dixiecrat state legislatures in Alabama, Mississippi, Arkansas and Tennessee, when such occurs, can be said to be the final conclusion of such a transformation. As of today, it is not yet complete; but taking 1928 as the beginning and 1972 as the median, we can expect it to be completed around the year 1972 + (72-28) = 2016, assuming symmetry around the trough of polarization and partisan power.