The Democratic margin has been 12 percentage points better, on average, than the partisan lean in each race. Sometimes this has resulted in a seat flipping from Republican to Democratic (e.g. in the Alabama Senate face-off on Tuesday or Oklahoma’s 37th state Senate District contest last month). Sometimes it has meant the Democrat barely lost a race you wouldn’t think a Democrat would be competitive in (e.g. in South Carolina’s 5th Congressional District in June). Sometimes it’s merely been the case that the Democrat won a district by an even wider margin than you’d expect (e.g. in Pennsylvania’s 133 House District last week).

The point is that Democrats are doing better in all types of districts with all types of candidates. You don’t see this type of consistent outperformance unless there’s an overriding pro-Democratic national factor. … So even if Democrats do 7 points worse in the national House vote than the average swing so far suggests, they’d still win the national House vote by 9 points, which would likely mean that they reclaim a House majority next year.