Why is Tester doing so much better than Heitkamp?

[Skye]

1) Rosendale is a weaker opponent than Cramer
2) Montana votes significantly to the left of ND
3) Tester is a more entrenched incumbent than Heitkamp
4) Heitkamp is facing a fellow congressional incumbent who won 69% of the vote in his last statewide election, in contrast to her 50%.

Pretty much. Also, the changing demographics of ND have made it far more hostile to Democrats than in the past.