North Dakota.
Worst case scenario for the GOP is losing Arizona, Nevada, Texas, and maybe Utah if a Trumpists is nominated there. If Trump wins this year, I'd say 2018 is D+2(+ AZ, NV, TX; - ND).
Donnelly is dead in the water unless Murdock runs again. McCaskill MIGHT survive, but I'd definitely bet against her at this point. Same for Heitkamp. Tester/Manchin, assuming that Trump is unpopular (which cannot be counted on), should be okay as long as they don't say anything stupid.
If Trump is popular, the above five lose, and keep an eye on Brown/Nelson/Kaine/Baldwin/Casey. Henrich and Menendez could theoretically lose, but should be fine.
As far as Dem pickups, Heller could go down, but Flake is probably fools' gold. Texas won't be competitive until 2030 period.
