it's conceivable for Tester and Donnelly to win by wider margins in 2018 than they did in 2012, but the chances of that aren't really calculable until we see their opponents and how (un)friendly one thinks the national environment will be for the GOP.
I think that's a stretch. Bayh (who was like... the most popular, unbeatable IN Democrat ever) lost by 10 points in 2016 after being up double digits in initial polling. I don't see Messer or Rokita doing worse than freaking Mourdock, who talked about rape and ran in a massive Democratic wave year.
Tester isn't going to win by more than 5, I'm 99.9% sure of that. I fail to see why so many people here believe he is incredibly popular or has so much crossover appeal? His voting record isn't very moderate either.
Even if Heller wins in 2018, he'll be gone after 2024 for sure, so I don't think spending millions to save this seat in 2018 is worth it in the end. But maybe NV will buck the trend again, who knows.