In our piece Monday detailing the death of the political middle, we blamed two main factors for the movement of Congress toward the ideological poles: closed primaries in which the nominees are chosen by the most liberal/conservative voters and redistricting, which over the last two decades has largely worked to create as many safe districts -- for both parties -- as possible.

Emory University political science professor Alan Abramowitz wrote us to take issue with putting the blame for the death of the middle on redistricting. (Our friends at the Monkey Cage Blog took issue with our blame game too.)  He wrote:

"If you compare the competitiveness of states and House districts today with the 1960s or 1970s, there are a lot fewer swing states and districts and a lot more safe or strongly D or R states and districts.  So the country is pulling apart as well as Congress and it's not a result of redistricting as the same trends are evident in the states as in House districts.  There is no incentive for members of Congress in safe or strongly D or R districts or states to try to appeal to voters outside of their own party.  And this trend is compounded by the growing party loyalty of voters so you don't need a very strongly D or R state or district to feel pretty secure.  That makes the primary the key election which reinforces the tendency to stick with your own party on votes."

And, to illustrate his point, Abramowitz included four charts comparing the relative competitiveness of states and House districts in 1976 and today.

Here's a look at how the states shook out -- by victory margin -- in the 1976 presidential election between Gerald Ford and Jimmy Carter:

And here is that same chart for the 2012 race between President Obama and Mitt Romney:

Here's a look at the presidential margins in House districts across the country in 1976:

And here it is for the 2012 election:

The point is clear. Yes, the political middle is dying (or dead). But the fact that so many states were decided by a close margin in the 1976 presidential race and so few states were decided by a close margin in 2012 affirms Abramowitz's argument that the disappearance of the center is more about a broader polarization in American politics than about how congressional lines are drawn.

Point taken.