I'll say +5 in the Senate (up to 54). Defend all seats, then pick up NV, AZ, TX, TN, and one of AZ-special, MS-special, or some other seat.
In the House, I think the outside edge of what is plausible would be quite favorable to Democrats. In the last 3 midterm elections in which one party had unified control of Congress and the presidency (so 2010, 2006, and 1994), the average pickup was 49 seats. As the
FiveThirtyEight article laid out, Trump is historically unpopular, and there are dozens of R+5 to R+10 seats that could go Democratic. So I'll say something like a 70 seat pickup is the upper-most range of what is plausible. TX-21 (Lamar Smith), NM-2 (Steve Pearce), and VA-7 (Dave Brat) are examples of seats that could go Democratic in this election.
The gubernatorial races could be a bloodbath of historic proportions. I think Democrats probably hold all their own seats then pick up NM, ME, IL, MI, and NV (D+5). FL, NH, WI, OH, MA, KS, MD, VT, and IA are all totally plausible as well (D+14 total). OK, GA, AZ, TN, TX, and even AL and SD are not completely safe, and I'll say at best Democrats pick up two of those states. So D+16 total.
