Manchin may be helped simply by the fact that he is not running for re-election under a Hillary Clinton presidency.
Pryor and Lincoln both had to run under an Obama presidency.
West Virginia seems to have nearly unlimited ticket splitting, unlike Missouri where it is more limited. Perhaps West Virginia is a Democratic state at heart that fears Obama and Hillary would destroy all their coal jobs and switch to renewable energy.
News flash: we will have to transition out of fossil fuels at some point.
Polls for 2018 elections mean nothing 23 months before the fact.