This Sept. 11, 2012, file photo shows the Cleveland skyline taken from Edgewater Park in Cleveland. Cleveland won the backing of a Republican National Committee panel all but guaranteeing the GOP's 2016 presidential pick will accept the party's nomination in perennially hard-fought Ohio. (AP Photo/Mark Duncan, File)

We're still waiting to see precisely how much of key portions of the so-called "Obama Coalition" stayed home last week, including black voters, Hispanics, unmarried women and young people.

But here's a pretty good approximation for just why Democrats struggled so much on Election Day.

According to numbers crunched by Gene Ulm of the GOP polling firm Public Opinion Strategies, turnout in the most rural one-third of U.S. House districts was down 34 percent from 2012. In the middle third -- think suburbs and exurbs -- it was down 38 percent.

And in the one-third most urban districts on the United States, it was down nearly half, 47 percent.

Why does this matter? Well, the first two categories are dominated by Republicans (214-71 in House races, as of today), while urban districts are dominated by Democrats (114-30). So when the turnout drop-off between those categories is that sharp, you can pretty much guess what it says about overall turnout between partisans on either side.

And while many key Democratic demographics vote in far lower numbers in midterm elections, seeing turnout drop off by nearly half in any one group is pretty rare. So this is a big deal.

In addition, for any Democrats running statewide, seeing such big drop-offs in your urban voter bases is going to make it much harder to win. See: GOP Govs.-elect Bruce Rauner (Ill.), Larry Hogan (Md.), Charlie Baker (Mass.), Rick Scott (Fla.) and Rick Snyder (Mich.) and Sens.-elect Thom Tillis (N.C.) and David Perdue (Ga.). All have big urban areas in their states and won in close races. Without that urban drop-off, they faced much tougher odds.

And while "urban" often connotes "black," and there was certainly a drop-off in that demographic (which in 2012 helped reelect the first black president), that's hardly the whole picture in the most densely populated districts. In fact, exit polls after the election showed drop-off among black voters from 2012 to 2014 was actually smaller than among young people and Latinos, for instance. The percentage of the electorate that was black (12 percent) was actually bigger than the last midterm in 2010 (11 percent).

We'll await the specific data, but the turnout drop-offs above pretty much tell the tale on what happened last week. The rest is merely getting deeper numbers.

Republicans won big on Tuesday night – as much as by who actually voted as who didn't. Here are the takeaways from the exit poll data. (Pamela Kirkland/The Washington Post)