Dave Wasserman, the Cook Political Report’s House analyst, says the most under-covered aspect of 2018 is that “a blue wave is obscuring a red exodus.” Republican House members are retiring at a startling clip — a trend that senior White House adviser Kellyanne Conway told me earlier this year was worrying her more than any other trend affecting the midterms.

What’s happening: There are 43 Republican seats now without an incumbent on the ballot. That’s more than one out of every six Republicans in the House — a record in at least a century, Wasserman says.

Why this matters: Just in the past eight months, the number of vulnerable Republican seats has almost doubled, according to Wasserman. Democrats need to win 23 seats to claim control of the House. Today, the Cook Political Report rates 37 Republican-held seats as toss-ups or worse. At the beginning of the year, it was only 20.