(Note: There was a special election in the very Democratic CA-34 which Democrats did win)
Looking at the chart above things aren’t that bleak. They beat the Trump Clinton margin by 14.5 points in three of the four races. Donald Trump won 230 congressional districts, but he won only 159 of them by more than 14.5 points. If they were to do 14.5 points better in 2018 Democrats would take the House in a landslide. The problem there is the one district they didn’t do 14.5 points better than the Clinton margin was GA-6. They only needed to do 1.6 points better and actually did worse. If they don’t beat Clinton’s margin they aren’t going to win many districts at all.
In three of the four races they improved on the 2016 congressional margin by 17.3 points. Republicans only won 190 districts by more than 17.3 points. If Democrats close the 2016 margins by 17.3 points they’ll win 245 seats. That’d be a smaller landslide but still a landslide. In Montana-AL, improving on the 2016 congressional result by 17.3 points would’ve won them the seat. That was the one district they didn’t improve by that much and didn’t win it. If they close the gap by 9.6% in all districts they’ll only net 13 seats in 2018.
The good news for Democrats is that they did beat the 2016 congressional margin dramatically in every district. The bad news is that these were all open seats. Incumbents are much harder to beat than winning an open seat. As of now, there is only one open seat in a competitive district, FL-27. That should change but right now Democrats will have to beat a lot of incumbents to take back the House.
In 2006 and 2008 Democrats won 56 Republican seats. Of those 20, 36%, were open. A number of the Republican incumbents in 2006 were plagued by scandal. If Democrats don’t get more retirements (or scandals) they’ll have to beat a lot more incumbents than they did in 2006 and 2008.
Who are the vulnerable incumbents? Is it the 24 Republican districts where the GOP candidate won by 12.8% or less? These four weren’t among those districts. Is it the 23 Republican held districts Hillary Clinton won? Again she didn’t win any of these. She did, however, come close to winning GA-6 and Democrats will have to win a 24th district to get the majority. Of course Democrats aren’t going to win every district Clinton won, even in a wave. They didn’t win every district Barack Obama won in 2008 either. They will have to beat incumbents in districts like GA-6 or MT-AL.