The flipside, of course, is that there are hundreds of much smaller media markets around the country where the Democratic brand has declined precipitously; one very typical example is the Ottumwa, Iowa, media market, where Dukakis outright won, 51-48, but where Clinton lost 64 to 31 in 2016. That may not matter that much at first glance when the New York market has nearly 150 times more votes than the Ottumwa market (8.4 million versus 57,000 in 2016), but when that same thing happens in dozens of similar rural markets, that’s the stuff that “death of a thousand cuts” is made of.
To be better able to see this phenomenon at a glance, take a look at the second tab on the spreadsheet, where I’ve consolidated all the numbers down to a one-year Partisan Voting Index number. “PVI,” as it’s known, is a concept created by Charlie Cook that measures the relationship between the two-party vote share in a CD (or any other geography), and the two-party vote share nationwide. (For instance, if Hillary Clinton got 50 percent in a market, while getting 48 percent nationwide, that would be D+2; if Barack Obama got 53 percent in that same market in 2012 while getting 51 percent nationwide, that would still be D+2. The advantage of PVI, compared with results expressed as simple percentages, is that it makes a time-series comparison much easier between elections that were close and elections that were blowouts, as well as with elections where there was a large third-party effect.)
As an example, let’s take a quick look at some of the major markets in Pennsylvania, as a way of illustrating the vastly different directions the various parts of the state have been taking. Here are the Philadelphia and Pittsburgh markets over the years since 1980, as well as the somewhat smaller Harrisburg market in the middle of the state, which has stayed pretty much the same during that whole period, and the even smaller Erie, Johnstown, and Wilkes-Barre markets, which, while not starting out as blue as Pittsburgh, have taken a similar downward trajectory. Taken together, they tell the story of what happened in Pennsylvania in 2016; the Philadelphia market, while staying dark blue, didn’t become even bluer in the way that the Clinton camp had hoped, in a way that would offset the rest of the state becoming even redder:
You might be thinking “wait a minute, Pittsburgh is still a dark-blue city.” Well, the unit of analysis here is the entire media market; it doesn’t incorporate just Pittsburgh, or just Allegheny County (which is still very much a blue county, though somewhat less than where it was a few decades ago), but all of the market: the entire Pittsburgh metropolitan area, plus some of the entirely rural counties that are situated outside that metropolitan area but still watch Pittsburgh TV affiliates. That R+6 is largely because of quickly-Republican-trending places in the exurbs like Westmoreland County and Fayette County. So you’ll see other instances where a major city reads as “Republican-leaning,” because it includes not just the city but its dark-red surroundings (like, for instance, St. Louis, which is currently R+4, thanks to the rural parts of eastern Missouri, or Nashville, which is R+14, because of the large and deeply-evangelical rural swath surrounding its metro area).
Finally, one additional benefit to presidential results-by-media market data is that sometimes you’ll see statewide poll data broken down by media market in the crosstabs. Looking at presidential results-by-media market data can help you assess whether the crosstabs pass the smell test. To zoom in on that, check out the third tab on the spreadsheet which breaks the 2016 results down by each state’s share of each media market.
In other words, many media markets cross state lines, and in some of those cases, you find conservative suburbs across the state line from a major city on the other side of the river. One prime example is the Wisconsin portion of the Minneapolis media market; the exurbs on the other side of the state line in Wisconsin are much more conservative than the Minneapolis market as a whole, so you wouldn’t want to assume that the Minneapolis market numbers as a whole apply to a statewide poll of Wisconsin that included a subsample of the small portion of Wisconsin that’s in the Minneapolis market. Instead, you could look at the third tab to see that while Hillary Clinton won the Minneapolis market as a whole by a 47 to 44 margin, she lost the Wisconsin portion 37 to 57.