As the government looks set to go into shutdown mode, thanks to the inability of Congress to pass a budget, it's worth looking at the roots of DC's divides.
The ongoing political crisis in the U.S. has many origins (the bad economy, gerrymandered congressional districts that punish deviation from ideological purity, the rise of the right within the GOP etc.).
One difference is demographic.
Conor Sen put together a fascinating chart that looks at congressional districts two ways.
The vertical axis shows a congressional district's population density (the number of people who live in each square mile).
The horizontal axis shows the district's Cook PVI score, which is really just the proclivity of the district to vote Democratic.
The pattern is clear; congressional districts with low population density (rural) are much less democratic-leaning. As the density rises, the inclination to vote D rises fast.