This election really is something. You could've argued that Dems did better because of low turnout or that Trump's base didn't show up -- but nope, this was presidential level turnout, with Dems turning out 60 million votes, and even WITH Trump's base being awakened, they nearly took the house by 10 million votes. That's incredible.
Which leads me to think that this election is some indication of 2020. You couldn't argue that 2010 meant the defeat of Obama since the turnout was pretty low, but this election signaled mass enthusiasm in the opposition with the Democrats beating the GOP by nearly 10 mill in such a turnout.
Essentially, the Dem candidate in 2020 needs to live in PA, WI, and MI. Based on 2018, we have a good starting point.
Yes, keep in mind that if this had been a presidential election with Trump vs generic D, Trump would have lost.