Which Democratic senator will face the toughest challenge in 2018 if running?

[McGovernForPrez]

McCaskill > Donnelly > Brown > Manchin > Tester > [...] > Stabenow > Heitkamp

Lol@Heitkamp winning in this poll. It's more likely that she cracks 60% than that she is the most vulnerable Democrat.

I think Heitkamp is relatively safe, but I can't say that she's safer than Stabenow.

[McGovernForPrez]

No way is Heitkamp safer than Stabenow (or other swing state Democrats, for that matter.) Unless her opponent pulls a Todd Akin, she's never going to be safe. There's little evidence that partisan gravity doesn't exist in North Dakota, but is an unstoppable force in Missouri and Indiana.

ND is a retail politics state, Heitkamp is more popular than Donnelly or McCaskill and also has very high name recognition. I mean, my point was more that both Heitkamp and Stabenow aren't going to lose unless the Democrats' floor is falling out from underneath them and 2018 is a GOP wave. Heck, am I the only one who thinks that the first polls will show her up something like 25 or 30 points?

Donnelly and McCaskill won against Republican candidates who basically imploded and made them look like moderates, while Heitkamp cracked 50% against a non-Akin Republican.

Popularity only gets you so far in hostile territory for your party, as Scott Brown proved in 2012, and Heitkamp will still have a tough time convincing swaths of Republicans in the state that she isn't basically a partisan Democrat. Sure, she has some crossover appeal, but it's not the same state it was in 2012, either. I could definitely see Heitkamp losing even if Democrats are having a good year, since ND is hardly a bellwether state. Early polls might have her well ahead, especially if she doesn't have an obvious opponent, but we should know not to trust polls this far out.

Donnelly won by 6% against an opponent who only made his comment very late in the race, which might have limited its effect on the race (polls did show the race to be quite close before then), McCaskill won by 16%, while Heitkamp won by just 1%. Yes, her opponent wasn't Akin, but Rick Berg was still not a good candidate. I don't think the 2012 results are the best evidence that Heitkamp is less vulnerable than other Democrats.

Scotty isn't a fair comparison. He only won the initial race because of special election dynamics, and anti-Trump backlash. With presidential level turnout he was basically DOA.