Democrats have outperformed in each special election since President Trump took office by an average of 8.5 points, according to the Cook Political Report's Partisan Voter Index, which measures the partisan tilt of each district based on the two previous presidential elections.

Why it matters: That's a large enough margin for Democrats to take back the House, says Cook's Dave Wasserman. And by a lot — he says they'd need to outperform PVI by just 4-5 points to win in November.