https://en.wikipedia.org/wiki/United_States_Senate_elections,_2020#/media/File:2020_Senate_election_map.pngHere's the map (though we actually don't know which party the Minnesota incumbent is pending the special election next year).
Its not a necessarily great map for Democrats, but they are almost 100% on offense. Only problem spot is Alabama. Minnesota, Virginia, Michigan, and New Hampshire could be competitive, but I doubt Trump wins any of those states and I see no reason why incumbent democrats would lose, save for Mark Warner or Gary Peters touching some kiddies. Maybe Sununu runs in New Hampshire, or Snyder in Michigan. But, bottom line, all incumbent democrats are heavily favored save for Jones.
On the R side, they have a bit of a numbers problem. They don't have a lot of blue-state Rs, but just a lot of extension. Gardner in Colorado is DOA, but Tillis, McConnell, Perdue, and Ernst will all have competitive races. Strongly favored, but still slightly vulnerable include Collins, Capito, and Daines. And there's plenty of moderate to deep R states where a scandal a la pedophilia could make it competitive.
Early prediction for Democrats:
'18: +Arizona +Nevada -Missouri -Indiana
'20: +Colorado +North Carolina -Alabama
So a 50-50 split at the end of the decade with probably a Democratic vice president assuming office.
