a Moderate Democrat from Montana would certainly gain the attention of Moderate Republicans as well as Independents...also, let's not forget the 'baggage' I mentioned that any Republican candidate would have to answer for.
Tester could have a problem with the far left segment of the Democratic Party, but then no candidate is going to be everyone's first choice.
Again, it's a bit early to call favorites...even a 'dark horse' could win the nomination...look at Lincoln.