As people have said on this thread and elsewhere, the two biggest motivations for voting are fear and anger.
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I think it pretty much goes without saying that Donald Trump's core voters are both angry and afraid. They're genuinely afraid that their way of life will disappear; they're genuinely angry about the condescension and dismissal that they (often accurately) perceive The Others are directing toward them. DJT's core voters *will* vote; they'll vote for him; and they'll come pretty close to straight-ticket voting for GOP candidates for all other positions.
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Disgust, by contrast, is not a particularly strong prod to get people to the voting booth: People who are simply disgusted by Candidate X are perhaps just as likely to stay home as to go to the polls and vote against him/her.
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In 2018 the Dems were able to mobilize a good level of fear in the electorate, particularly fear about losing health care (or having it be unaffordable); and there was a fair level of anger, too. The question for the DNC is whether they can sustain those levels of fear and anger in 2020. There is such a thing as rage-fatigue: Eventually one's capacity for anger is exceeded, and many commentators have already remarked this in the Age of Trump.
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So it really boils down to fear, and right now it seems that the GOP is making the better going in that regard. There are a lot of fearful people in the country: afraid of religions not theirs, afraid of languages not theirs, afraid of skin hues not theirs. What issue(s) can the Democrats use to spark those same levels of fear? . . . Because simple disgust with Trump will not be enough to win.