Here at Daily Kos Elections, we’ve been keeping track of retirements from the House all year long, because a House seat without an incumbent is easier for the opposite party to win. Our spreadsheet has now been updated back to 2006, and includes seats that will not have an incumbent on the ballot in November for any reason, including losing a primary,
Comparing the pace of open seats on the Republican side to 2008 (which had the highest number of Republican open seats in decades) and 2006 (when Democrats won back the House) shows 2018 could easily beat 2008. Indeed, there are typically 10-15 more seats that open up on the Republican in the election year itself, as you can see in this cluttered figure that shows all the data back to 2006.
Democrats, on the other hand, aren’t going to be breaking any records this year, on either the low side or the high side. Come below to see the data for the Democrats, and Republicans, stretching back to 1984.