Conor Lamb has apparently won a very narrow victory over Rick Saccone in the special election in PA18.
As I pointed out on Monday, the federal special election held before Pennsylvania 18 showed Republicans in a poor position. In the average of seven special elections before this one, Democrats were outperforming their partisan baseline (based off the previous two presidential results in the district) by 16 percentage points. In Pennsylvania 18, Lamb, the Democrat, outperformed it by 22 percentage points -- a little better than average and essentially matching what they did in the Kansas 4 special election in April 2017.
If the past relationship between special election results and the midterm results holds in 2018, Democrats are in a strong position to take back the House in 2018.
I collected data on each president's approval rating at this point before each midterm election in Gallup surveys since 1946. Trump's 39% approval rating at this point projects that the Republicans will lose the House popular vote by 10 points. (There's a fairly wide margin of error in this estimate given that we are still have a lot time before the midterm.)
If the 10-point estimate is right on, Republicans will probably have a net loss of between 30 and 40 seats. That is far more than the net 23 seats they need to lose (if Lamb wins) for them to lose the House.