You've no doubt seen a map similar to the one below showing the breakdown of party control by congressional district.
Closer analysis will reveal that Democrats are big in Urban districts and Repubilcans more rural and suburban areas. The Economist article, from which this map (and the two following) came, says:
Run down the top 30 incorporated places in America; the only ones represented by Republicans are Fort Worth, Texas and (as of January) Columbus, Ohio. Every other major city in Texas, and in fact every other major city in the South, is represented mainly by Democrats.
But then it shows two other eletoral maps ... one for Germany and one for France: