Daines could have been vulnerable in the 2018 mid-terms, but he'll probably win in 2020 unless Trump loses in a 1984-style blowout
I don't see why he would be more vulnerable in a midterm year than in a presidential year.
I'd argue that the "anti-president's party" effect of mid-terms is generally going to be stronger than the "pro-Republican" effect caused by lower turnout. The last three mid-terms have been disastrous for the president's party. If not for 9/11 and the Clinton impeachment, I suspect that the last six mid-terms would have been disastrous for the president's party.
Also, discounting the case of Ted Stevens, the last time Democrats knocked off an incumbent in a red state was the 2006 Bush mid-terms (in Missouri and Montana).
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