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How Pennsylvania ‘turning red’ helps explain modern presidential politics

On Monday, NBC's Jacob Soboroff shared a report from his visit to Aliquippa, Pa., with the hosts of "Morning Joe" -- a follow-up to a recent Quinnipiac University poll showing a close general election race in a state that hasn't voted for a Republican presidential candidate since 1988.

There's a snippet of that report that we should focus on, the map of how counties in the state of Pennsylvania have gotten increasingly red (that is: Republican) over the past few presidential cycles.

(Aliquippa is in Beaver County, the outlined county on the western edge of the state in the visual above.)

The red-blue presidential election map came to prominence in the wake of the 2000 election, when it was used to explain the muddled path George W. Bush took to victory. One way that was done was by presenting a map of the results by county, showing a mostly red United States thanks to Bush's margins of victory in the vast rural areas that make up much of the country's land area. The Democratic Party is heavily centralized in the country's urban areas, as a look at a map of the House makes clear. So those 2000 maps ended up making the country look a lot more Republican than it really was.

Such is the case with Pennsylvania.

Let's do a more complicated version of the map above, adding two factors. First, let's use a slightly richer scale for how red or blue each county is. Second, let's include population.

The circles are scaled to the relative percentage of the population each county constitutes relative to the state's overall population in the year of the election. The bigger the circle, the more people.

In case you hadn't noticed, pay attention to the lower right part of the map -- the southeastern region around Philadelphia. (The county that stays blue on the western part of the map is Allegheny County, home to Pittsburgh.) It's there that you can see noticeable changes, as the counties surrounding Philadelphia swell slightly over time. And as they do so, they get bluer.

Put another way: Counties that grew in Pennsylvania over time also voted more Democratic. And Philadelphia itself, the most populous county in the state, moved much more to the left.

This may be a clearer way to see that trend. Since 1984, when the state voted for Ronald Reagan (as did just about every other state), most counties shifted to the left (more Democratic) and then back right. But Philadelphia County kept moving left, even as it became slightly less populous as a percentage of the state. Allegheny County -- the urban heart of Pennsylvania steel county, of which Aliquippa is part -- hasn't changed much politically as it has become a smaller portion of the state.

Notice too that the most populous counties in the state are also the most Democratic, consistently. Again, this is partly a function of the centralization of Democratic voting in cities, and it's partly a function of the counties around Philadelphia -- Bucks, Montgomery, Delaware -- growing and (over time) moving left.

The story of Pennsylvania getting "redder" is really a story of how politics in the country have changed. Let's isolate the 1984 and 2012 frames from that animation above.

In 1984, Allegheny and Philadelphia counties were Democratic, but the rest of the counties were a mix of red and blue. By 2012, most of the more populous counties were blue and most of the less-populated ones were red.

Pennsylvania could go for Donald Trump this year, as FiveThirtyEight's Dave Wasserman explained last week. But focusing on how voters in a small county might meander across party lines misses the story of what's really happening in the Keystone State. And, therefore, misses the story of what's happening to American politics overall.