States like MT and ND behave differently in down-ballot races, and there's a reason why Republicans are struggling to win Senate races in those states.
I am beginning to think that this no longer matters. And the Republicans have not struggled to win Senate seats in those states. They nominated a weak candidate in 2012 and there other others who would have won it easily. As for MT, Rehberg ran a terrible campaign.
Also I would note that this is beginning to change. A lot of these states became more D friendly during the Farm Crisis of the 1980's. That is how Dems ended up with both ND Senate seats, both SD Senate seats, both NE seats and Iowa became a D leaning swing state.
If there is one trend that I think is long lasting in the Trump era, it is the undoing of the impact of the farm crisis and the returning of these states to solid GOP form.
And what you said about ND and MT, also use to apply to NE and SD. But that is no longer the case. ND was one of Trump's best states and no one saw it coming. Many figured it would be high 50's. We are in an era of heightened partisanship as well and so it might just be the case that as long as the Republican nominee is semi-competent, that the state just elects them by default and the only thing that Heitkamp's personal touch and superior campaign structure accomplish, is to just keep it close.