Quite unfortunate that Democrats can't and won't make a play here just four years after they still held that seat.
Democrats would be wise to at least run a credible candidate, that could benefit Tester in the Senate race. Of course, nobody wants to be a sacrificial lamb, and besides, Tester is unlikely to win reelection anyway. Still, you're right - it's sad that Montana is lost.
Unlike Joe Manchin, there is a very narrow path to victory for Tester. It would require the Montana GOP completely screwing up the Senate nomination, plus it would require a relatively close Presidential race in Montana.
For what it's worth, with Gianforte bowing out of the Senate race, I think Tester's path to victory is actually open. He would have been DOA against Gianforte. He probably has to hope he's facing Rosendale or Zinke, though to have anywhere close to a realistic chance at reelection, though.
Tester has a robust approval rating unlike Manchin and Montana isn't West Virginia. I wouldn't say his path is just "very narrow". It's a tossup, especially against Rosendale.
