There is one key way to increase the margin by which you win your primary: Be the incumbent. Granted, it’s hard to be the incumbent until you’ve already won a race (although not impossible!). But it’s still about the most sure-fire way to ensure you’ll win handily. That’s still very much true, but at least things are getting more competitive. With the 2014 primary season concluding Tuesday, we pulled data on the five most recent primary seasons to see how much that incumbent effect changed over time. The good news: Races are getting closer! The bad news: Only under a very particular subset of conditions. And they’re not getting very much closer. But, still! The average margin of victory for winners of House primaries in 2014 topped 77 percent; for the Senate, it was nearly 70.
But that’s in large part because a lot of people win uncontested primaries, giving them 100 percent of the vote. And the number of uncontested House races has dipped a bit recently, but is still high.
The number of uncontested Senate races is lower, in part because they’re statewide. And, in part, because there are more minor-party primaries on the House side. (Yes, we included third-party primaries; this is America, after all.)