My best plausible result for democrats is D+26 in the house and D+2 in the senate which would have them holding all their senate seats while flipping republican senate seats in Arizona and Nevada. This scenario would give them narrow majorities in both houses of Congress and would be a utter disaster for republicans, especially if trump still managed to win reelection in 2020 which would set them up for another midterm wipeout in 2022 followed by a landslide presidential defeat in 2024 and possibly decades in the electoral wilderness. The reason I don't believe democrats can have the massive house gains others are predicting is because their pathway back to the majority is heavily dependent on winning as many Clinton-Republican districts as possible (theirs 23 of them), holding on to all the Trump-Democratic districts (theirs 12 of them) plus winning a few marginal Trump-Republican districts. I just have a really hard time seeing democrats winning a bunch of districts trump won by double-digits with the exception of the few that are currently held by democrats.
If democrats are winning back the senate, they will get far more than f'ing 26 house districts lol. Do you even know the difference between the house and senate maps? The house maps are far more favorable to dems than the senate maps. There's literally 23 house districts that voted for Hillary Clinton. Meanwhile dems have to defend a ton of senate seats that Trump won.
Dems probably need to gain at least 40 house seats before they can take back the senate.