Just how strong are the GOP's gerrymanders? Daily Kos Elections' median district scores explain

In 2010, as Daily Kos readers are well aware, Republican swept to victory in Congress, taking back the House and making big gains in the Senate. At the same time, they also captured many governorships and legislatures, giving them control of the decennial redistricting process. With the map-maker's pencil firmly in hand, these new GOP majorities were able to draw district lines that favored them both at the congressional and legislative levels.

Scholars and pundits have debated exactly how much these maps are tilted in the Republicans' favor, but at Daily Kos Elections, we've developed one metric that's methodologically rigorous and highly illustrative of the GOP's advantage, relying on the "median seat" in each legislative chamber or state congressional delegation. The numbers, as we will explain, are starkly in favor of the Republicans.
Previously, we calculated the results of the 2012 presidential election according to every congressional and legislative district in the nation. That allows us to determine every state's median congressional district and every chamber's median legislative seat. But what do we mean by this median seat? In state legislatures, the median seat is the theoretical halfway point in each chamber whereby, if Democrats held every seat bluer (or Republicans held every seat redder), they'd control a majority of seats in that body.

For instance, in Ohio's state House, Mitt Romney won the median seat by a 51.3 to 47.6 margin. That means that if Democrats want to retake the House under the current map, they'd need to win all the districts that suported Romney by a 3.7 percent margin or less—at a minimum. Things rarely work out so precisely, though: Since Republicans are likely to hold a few seats that are bluer than this threshold, Democrats would have to capture a some redder seats as well. That's no easy task.
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