Another thing to keep in mind is that Democrats will be really motivated to win back the Senate in 2020 or at least make significant gains, which is why they will recruit top tier candidates everywhere. There is no way Daines, Ernst, Sullivan, etc. will easily cruise to reelection. Republicans are deluding themselves if they think Daines has the advantage just because he is an incumbent, for example.
And they do have a pretty decent potential candidate in those states - Montana's Bullock, Alaska's Berkowitz, North Carolina's Jackson, Georgia's Yates... etc. etc.
