This race is a pure Toss-up, and I do think a lot of people are seriously underestimating Bullock's chances here. While I could see some last-minute shift/momentum (as a result of some combination of trends catching up with voting patterns, undecideds breaking heavily R, polls slightly underestimating R strength, etc.) costing Democrats like Greenfield and Cunningham whose fortunes hinge on a very Democratic-friendly environment and/or Biden overperformance at the top of the ticket, Bullock has a high built-in level of support and stellar organization which won’t be easy to overcome regardless of the result at the presidential level.
So it looks like there is no Green or Libertarian candidate on the ballot this cycle. Just Bullock and Daines. Mono y mano.
No write-in either, as far as I'm aware. "Some other candidate" voters will be spoiling their ballots.
It’s like FL-SEN 2018: There’s a write-in option for every race, it’s just that there’s no serious write-in candidate/campaign (the filing/registration deadline for write-in candidates passed in September), so yeah, it would be equivalent to spoiling your ballot.