By Cohn’s estimates, 13 seats held by retiring Republicans are competitive. In our example above, it’s a Republican retiring from a district won by Mitt Romney in 2012 — who then sees a Democrat win the seat. In midterm elections, that happened nine times (out of 105 races in which the retiree’s party won the presidential contest in the district). If you’re curious, there are four Republicans retiring before 2018 who aren’t seeking a different office and who represent districts won by Hillary Clinton in 2016. There is one Democrat retiring who represents a district won by Donald Trump.

Source: Washington Post January 11, 2018 21:11 UTC