Incumbents have historically crushed elections,re-elected on average 92% of the time in every major election cycle since 2006. Republicans lack an incumbent in more than 40 races this year.
Why it matters: Open seats are more likely to flip than those with an incumbent on the ballot. Republicans have more open seats this year than at any point in the last 12 years, which could make them especially vulnerable in November.
Wave election years make things even worse for keeping control of an open seat.