Story highlights Only 16.5% of districts are even marginally competitive

Incumbents have drawn safe districts for themselves for last 20 years

(CNN) It's no secret that Congress doesn't get much done these days. And that partisanship and polarization are at record levels.

Why? Well, lots of reasons. But this new chart via the Cook Political Report (a former employer of mine!) reveals a big part of the problem.

The decline of the swing seat via The Cook Political Report

What the chart shows is the almost complete disappearance of competitive House districts over the past two decades. In 1997, there were 164 congressional districts that performed somewhere between five points more Democratic and five points more Republican than the nation as a whole. In the 2016 election there were only 72 of those seats. (For more explanation of the Cook Report methodology on swing seats, check this out.) That's 16.5 percent of the entire House of Representatives who sit in potentially competitive districts.

Why the change? Politicians -- Democrats and Republicans -- tasked in most states with crafting congressional lines once every decade -- have prioritized protecting themselves over creating competitive districts. The 2001 and 2011 redistricting processes were hugely driven by a mission to draw incumbent-friendly maps. Only in a handful of states -- such as Iowa, which uses a nonpartisan entity to draw its congressional districts -- do you see any sort of real competition.

As the Cook Report's David Wasserman notes, however, redistricting isn't the prime culprit for the decline of swing seats. The true "blame" lies with something commonly referred to as self-sorting. Since 1997, we as a country have tended to prize living in and around people who share our views and values. No amount of line-drawing can change that fact. We have redistricted ourselves.

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