Why have Dems been able to win US Senate Elections in Montana and North Dakota?

[hopper]

Why have Dem been able to win US Senate Elections in Montana and North Dakota for so long? Those states have been red for so long on the Presidential Level. For example Romney carried ND by like 15 points and Heitkamp won  the US Senate Race won by 1 % point in 2012. I would expect the Republicans to win US Senate Elections like they do in the Deep South in MT and ND. Were past ND US Senators able to localize their races like Conrad and Dorgan and not run on the Dem National Platform which is liberal compared to the ND State Party Platform I  would think. I just think of ND and MT as Center-Right and not center left which Conrad and Dorgan are/were.