Now we've taken it a step further. This is how every single county in the United States has voted vs. the national average since 1960.
The redder the red, the more Republican the county voted than the rest of the country. The bluer the blue, the more Democratic it voted. In 1960, 1968 and 1992, there are some counties that were a flat red. They voted against the Democratic winner and for third-party candidates. (Harry Byrd, George Wallace and Ross Perot, respectively.)
By far the most interesting thing about this animation is how the density of the colors increases. In the late 1980s, most counties were fairly bipartisan. By 2000, there are a lot of very strong red counties — a trend that increases. Keep an eye, too, on Appalachia. Until 2008, it's a pale blue. Then it quickly grows red.
Another way to look at it is to compare the 1960 vote in each county against its 2012 vote (or, in some cases when the counties didn't exist back then, the first available data after 1960). You can zoom and scroll and click and so on on this map.
As expected, the South grew much, much more Republican. Except Mississippi. In 1960, most Mississippi counties didn't pledge electors to the Democrats. In 1964, the state strongly — strongly — favored Republican Barry Goldwater. By comparison, the margin of opposition to President Obama versus Mitt Romney pales. The state voted much more Democratically in 2012 than in 1964 — even though it went for the Republican.
We pulled out two other sets of data. The first was a look at the counties that host six major American cities. Nearly all grew at least somewhat more Democratic over time — and some, like Los Angeles, substantially so. (The dashes don't mean anything other than to make it easier to read.)
We also identified the two counties that changed the most and least between 1960 and 2012. Wyoming County, N.Y., has voted 30-plus percentage points more Republican than the rest of the country in basically every election since John Kennedy first won. King County, Tex., however, has gotten remarkably more conservative.
And now the very fun part. Here are interactive versions of the maps that we used to create the animation above. Explore away.