I think the big question is not how much people hate Trump, but how polarized people are. It's possible that even in an absolute Democratic tsunami year, people in WV and MO and places like that simply aren't willing to vote a Democrat to federal office. However it's also possible that that isn't the case. We'll wait and see, but my point is that Trump's popularity or lack thereof is not the only factor at stake here.
I agree, I think polarization has increased so much over the past 10-15 years that certain states are going to be insulated.
I think it's quite possible that Trump's approval could be 25% as of November 2018 (I suppose it can also be 50%, and one can't predict the future but 25 is more likely), but that 25% could be concentrated in a few states, some of which have Dem senators up for re-election.
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