Actually, PPP's last poll had Daines up by 17, not 15. Also,
here's the PPP article on the race that notes Daines' decline in GOP and Indie support and Walsh getting a more unified, Democratic base. Daines' approvals have also dropped underwater to 39/40 disapprove from his last 41/33 approval. Walsh is 38/37 approve.
Regarding this, Daines' drop in job approval and poll numbers can be attributed to Walsh's sharp attacks on connecting Daines' to the government shutdown as well as his attempts to criminalize abortion. It's worth mentioning to that Walsh's name ID numbers since November have gone from 53% to 26% which in large part is due to all of those pro-Walsh ads so far talking about his record in the military and DC.
At least for Dems, we can start to feel a lot more optimistic about holding this seat, but for now, it still leans Daines. I think we can agree that we have a better chance of holding MT than WV at this point.