Democrats may already be 35% of the way to a House majority
With the retirement announcement of Pennsylvania Republican Rep. Ryan Costello, there are now eight open seats currently held by Republicans in districts that Hillary Clinton won in 2016.

And there's a good chance the Democrats will win them all, bringing the party 35% closer to the 23 seats it needs to flip to win control of the House.
That's because the opposition party just doesn't lose seats like these eight in midterm elections.
Going back to 1994, the president's party is batting 0.000 in seats like Costello's, where there will be no incumbent on the ballot and where the president's party's candidate won in the previous House election but the opposition party's presidential candidate won in the previous presidential election. (Hat tip to the Cook Political Report's Dave Wasserman, who first pointed out this statistic.)
You'd have to go all the way back to 1990 for such a seat to be held onto by the president's party, according to a dataset put together by political scientist Gary Jacobson. That year, Republican Jim Nussle held onto a Republican seat in Iowa's second congressional district by just a point. https://www.cnn.com/2018/03/27/politics/enten-costello-open-seats/index.html