By Geoffrey Skelley:

Incumbent politicians have moved further toward the political extremes in recent elections partly because they are worried about a primary challenge. But studies suggest that the primary electorate itself isn’t any more ideologically extreme than the general electorate. Rather, the bigger problem is the decline in competitive congressional districts. Only about 1 in 6 congressional districts were “swingy” in the 2020 general election, compared with roughly 2 in 5 in 2000.

The rapid decline in competitive elections isn’t because of our primary system, though. It’s due mainly to partisan sorting, whereby Democratic areas are becoming more Democratic and Republican areas more Republican — either because people are changing their attitudes to better match their party or they’re moving to areas where their preferences are already dominant. 

The upshot, of course, is that with fewer competitive districts, a primary is often more important than the general election, as it’s in this stage that the eventual winner is selected. That’s one big reason why incumbents fear a primary challenge even though few incumbents lose primaries — it’s the primary that increasingly matters for electoral survival.

I agree about the importance of competitive elections in creating incentives for candidates to appeal to a broader electorate. See my piece “Create More Competitive Districts to Limit Extremism.” The one point this 538 piece does not recognize is how little weight is giving to the importance of creating competitive districts — and not just when legislatures redistrict. Many reform proposals focus on other values and give no weight to creating competitive districts. Some of the reform proposals that have been adopted by voters, such as in Colorado and Arizona, do stress the importance of competitive districts. But others do not.