Since 1994, a sitting member of the House has lost renomination to a challenger 31 times, calculated the Daily Kos's Jeff Singer in 2014. (Make it 35 as of August 2016.) For perspective, that's out of some 2,000 elections over the past two decades.
Incumbent lawmakers have lots of inherent advantages: more name ID, more connections to donors and often a better understanding of the electorate they're trying to represent again. Plus, redistricting has helped match up seats with the lawmakers representing them. (Hey, those in power get to write the rules.)
As a result, you can count on one hand the number of House lawmakers who have been taken out in a primary in the past few cycles. Former House majority leader Eric Cantor (R-Va.) is probably the highest profile. He lost his primary in 2014 to a little-known conservative college professor, now-Rep. Dave Brat, catching the political world by surprise.
But it's not always a shock when incumbents lose their primaries. Incumbent primary losses are often individualized cases that require one or a combination of the following: Lawmakers who upset powerful interest groups [Rep. Tim Huelskamp (R-Kan.)], lawmakers facing criminal charges [Rep. Chaka Fattah (D-Pa.)], lawmakers who struggled with personal problems, or lawmakers who vastly misread or underestimated the political mood and are seen as out of touch.
Even Cantor's loss, when viewed in hindsight, fits into that last bucket. Voters complained he had become out of touch with his suburban Richmond district as he climbed the ladder of leadership, and some Republicans thought he took his reelection for granted. You could make a similar case for another shocking primary loss in 2012, when 24-year veteran Rep. Cliff Stearns lost his Gainesville, Fla.-area district to a veterinarian with no political experience.
There is one major caveat to this incumbents-are-hard-to-oust rule: Sometimes redistricting can work against lawmakers, especially when the courts order new electoral maps drawn to try to rectify illegal gerrymandering.
That often scrambles the map, pitting two lawmakers against each other or putting a lawmaker in a district with a different ideology. Primary losses where redistricting is involved happen with much more frequency. Of the four House lawmakers who have lost their primaries so far in 2016, redistricting was involved in two contests: Reps. Renee Ellmers (R-N.C.) — who had been endorsed by Donald Trump — and Randy Forbes (R-Va.).
Some Republicans were worried that 2016 would bring with it a new reason to fear losing in primaries: their party's presidential nominee. Specifically, establishment Republicans worried that Trump's outsider-fueled campaign would boost primary challengers trying to ride his coattails. (See Paul Ryan's primary challenger, Paul Nehlen.) In March, longtime Sen. Richard Shelby (R-Ala.) was the first Republican to share a ballot with Trump, and he spent some $5 million in his primary to avoid having a run-off with a little known challenger.
Shelby easily won. And so far, we haven't seen any evidence that we need to add a new rule to the list of why incumbents lose primaries. In six key Senate primaries, all of the establishment Republicans' preferred candidates have won.
Which brings us back to our original point: It's really, really hard to oust an incumbent lawmaker, even in this chaotic campaign.