The overall message of 2017 special elections is that Republicans are in troublehttps://www.vox.com/policy-and-politics/2017/6/21/15846464/republicans-are-in-trouble...
Finally:
You can define 'trouble' however you want, though. Trouble could mean losing the House, or barely holding it and maybe losing it in 2020, or it could mean holding the House but losing bigly in Governors races/etc.
But the point is that Republicans
significantly lost ground to Democrats in districts with all kinds of demographic profiles, showing that Democrats have already proven they can make up lost ground with working class whites while
also keeping their newfound gains with white college graduates. While everyone is bickering about losing special elections in deeply Republican districts, not appreciating the massive swings to Democrats would be absurd. We have had a number of special elections in legislative and Congressional seats so far, and it is enough to say that Republicans are likely facing a pretty bad midterm - that is, unless things get significantly better for Trump and Republicans, which isn't likely so long as Mueller is there.
Let's also keep something else in mind - it's not all about Congress. It's also about state legislative and gubernatorial races, both of which Republicans are overextended in. State politicians can sometimes avoid the heat of their national party, but not completely.
Anyway, good article. Worth a read.