If you want to understand the evolution of American politics over the past 100 years, you could do a lot worse than watching the GIF above, which shows party control by Congressional district from 1918 to 2012. The maps were put together by Jonathan Davis, a research aide at Arizona State University's Decision Theater. You can click over to mapstory.org for a full interactive version.

There are a million stories to tell here. You could talk about the explosion of Democratic control in 1932, coinciding with the election of Franklin D. Roosevelt and the ouster of a deeply unpopular Herbert Hoover. There's the brief flourishing of the Farmer Labor and Progressive parties in Minnesota and Wisconsin in the 1930s. Or you could look at the evolution of the Democratic party from South to North, particularly following the passage of the Voting Rights Act in 1968.

We talk a lot about political polarization these days. What stands out to me is the level of geographic polarization in the early 20th century. Take a look at the map from 1920, for instance.

The North and South are two strongly distinct entities, delineated by their politics. There are just a small handful of Democratic districts in the Northern states, and Republican districts in the Southern ones. Appalachia extends a long Republican arm into otherwise Southern territory. In many cases the party borders follow state borders - the Arkansas/Missouri border stands out, as do West Virginia and Virginia.

Now, contrast that map with the one for today's Congress.

Our political geography is much more fragmented. The story here is less North/South, and more urban/rural. Majority-minority Southern districts stand out as islands of blue in a sea of red. Democratic districts are concentrated largely on borders and coastlines, while the interior of the country is deep red. Maryland's Eastern Shore remains politically distinct from the rest of that state.

Last week an article in the New York Times noted that population shifts were turning all our politics national. This map provides one illustration of how that's happening. A hundred years ago, you could be certain that the politics of Oneonta, New York differed from the politics of Oneonta, Alabama simply by knowing their relative latitude. In today's fragmented political landscape, such certainties are a bit harder to come by.