Not only are Democrats trending upward with a ever expanding base by HISTORIC numbers as seen in the mid terms but the Republican base has retracted substantially and they arent gaining new voters. Anyone thinking the Republicans or Trump will win in 2020 is simply naive. Democrats should use the impeachment hearings as a cudgel to repeatedly expose and weaken the GOP well into the summer of 2020. The majority of the populace know the Republicans are corrupt liars now make that perception undeniable. Anything else saying different has no idea what they are talking about and are only interested in obfuscation and misdirection.
Nah, thinking they have it in the bag is part of what led to 2016. It's not a mistake Dems should repeat. Midterms aren't a good predictor of the next general election (see 2010/2012).
Trump will probably lose the popular vote by even more but could easily win again electorally. The Dem base may be growing, but the way Dem voters are distributed geographically is inefficient since they're mostly bunched up in urban areas. Sure everybody knows Trump and Republicans are corrupt, but a large plurality of voters don't care as long as he's antagonistic towards minorities and immigrants. As terrible as Trump is he's still an incumbent president and Dems need to assume they're the underdogs and try to turn out all the voters they can.
I completely agree about impeachment though, they should drag out the hearings as long as possible to maximize outrage when the Senate exonerates him.