I believe the final PPP poll in 2018 had Tester up 4, so yeah, this is looking very close. I imagine that most undecided voters are leaning toward Trump, given the state we’re talking about, and Bullock’s path always required winning over Trump voters open to voting for him. I’d give Daines a very slight edge here, but this race is far from over, and some of the “TN-SEN 2018 redux” takes from a couple of months ago aren’t going to look good.
90% of undecideds in this poll being Trump 2016 voters isn't such a crazy issue for Bullock when he and Tester both had to peel off huge amounts of them for their wins in 2016 and 2018. Not a favorable one, but not impossible.
To repeat what people have been saying here, this race will be down to the wire - it could be a real heartbreaker for Democrats (and Jon Tester, who probably never wants to work with Daines again).