Watch the Democrats’ hold of the South fall apart, in one map

Democrats used to dominate the South. Today, they dominate America’s cities.

The map below, uploaded on MapStory by Jonathan Davis at Arizona State University, shows the political transitions of congressional districts between 1918 and 2012. To get it working, zoom out and press play.

The map, in short, shows how America’s political lines shifted from a North-South divide to an urban-rural divide over the past century.

The new divide has real consequences for both political parties. As Vox’s Andrew Prokop explained, one of the major issues for Democrats trying to take control of the House of Representatives is the geography is simply to their disadvantage. Democrats tend to be packed into urban areas and small college towns, while Republicans sparsely populate wide regions all over the country.

It’s a lot easier to draw a congressional district map, then, that includes more Republican than Democratic districts. That’s one of the reasons Davis’s map begins to have a lot more red than blue after it moves from the era of the Democrat-controlled South to the era of Democrat-controlled cities.