In 1992, there were 103 members of the House of Representatives elected from what might be called swing districts: those in which the margin in the presidential race was within five percentage points of the national result. But based on an analysis of this year’s presidential returns, I estimate that there are only 35 such Congressional districts remaining, barely a third of the total 20 years ago.
Instead, the number of landslide districts — those in which the presidential vote margin deviated by at least 20 percentage points from the national result — has roughly doubled. In 1992, there were 123 such districts (65 of them strongly Democratic and 58 strongly Republican). Today, there are 242 of them (of these, 117 favor Democrats and 125 Republicans).
So why is compromise so hard in the House? Some commentators, especially liberals, attribute it to what they say is the irrationality of Republican members of Congress.
But the answer could be this instead: individual members of Congress are responding fairly rationally to their incentives. Most members of the House now come from hyperpartisan districts where they face essentially no threat of losing their seat to the other party. Instead, primary challenges, especially for Republicans, may be the more serious risk.
In the chart below, I’ve grouped the country’s Congressional districts into seven categories based on the results of presidential voting there from 1992 through 2012:
• Landslide Democratic districts are those in which the presidential vote was at least 20 points more Democratic than in the country as a whole. (For example in 2008, when the Democrat Barack Obama won the popular vote by roughly seven percentage points nationwide, these districts were those in which Mr. Obama won by 27 percentage points or more.)
• Strong Democratic districts are 10 to 20 percentage points more Democratic than the country as a whole.
• Lean Democratic districts are 5 to 10 percentage points more Democratic than the country as a whole.
• Swing districts are within five percentage points of the national popular vote margin.
• Lean Republican districts are 5 to 10 percentage points more Republican than the country as a whole.
• Strong Republican districts are 10 to 20 percentage points more Republican than the country as a whole.
• Finally, Landslide Republican districts are at least 20 percentage points more Republican than the country as a whole.
As these figures make clear, the number of swing districts has been on a steady decline since at least 1992, and the number of landslide districts on a steady rise. The year 2008 was a partial exception: the number of landslide districts rose slightly from 2004, but so did the number of swing districts. However, the polarization of Congressional districts became sharper again in 2012.