Harry Enten at 538 with a live blog post about why a close loss by Quist is meaningful. I'm just going to quote it verbatim:
We’ve gotten some questions and comments from readers to the effect of: What are you stupid? How can you say a Democrat losing by several percentage points good for the party?
It’s a fair question. At the end of the day, Democrats need to win Republican-held seats to take back the House. But Montana’s seat wouldn’t be a top pickup opportunity. There are 120 Republican-held seats that lean more Democratic on the presidential level than Montana’s. So looking at the Montana race by itself isn’t helpful if you’re interested in the national picture.
Instead, we look at how much better Quist is doing than we’d expect given the presidential lean in Montana. If he loses by say 8 points. It means he outperformed Montana’s default political lean — how it would vote in a presidential election that was tied nationally — by 13 points. There are 64 GOP-held seats that are 13 points redder than the nation as a whole or less. Now, Democrats likely won’t win all 64, but that’s why a mid-single digit loss for a Democrat in Montana suggests that a lot of seats are in play for them in 2018.