This enormous Republican advantage is likely to endure for the rest of the decade. It would take a five-point swing for Democrats to capture just one more seat. For the map's skew to disappear, Democrats would need to improve on their 2016 showing by 14 points — a bigger wave than Pennsylvania has seen in generations. And the map's tilt can't be explained by the clustering of Democratic voters in Philadelphia and Pittsburgh. In hundreds of computer-simulated maps, Democrats usually win nine or 10 of Pennsylvania's 18 districts. The state's underlying geography is thus close to neutral.