For now, Democrats are ferociously opposed to Trump personally and to his presidency. Their fervor could fade in the 17 months between now and the 2018 election—but probably not by much, because attacks on Trump, including calls for his impeachment, are likely to dominate Democratic campaigns.
A more subtle factor in 2018 is the role of the coalition that put Trump in the White House. He appealed to millions of working-class voters, many of them Democrats or independents or nonvoters in the past. He wouldn’t have won Pennsylvania, Michigan, and Wisconsin without them.
But it’s unclear whether they are casual voters attracted solely to Trump. Will they will turn out for Republicans in a midterm election in which Trump isn’t on the ballot?
Nationwide, the turnout is smaller in midterms (40 percent) than in presidential years (60 percent). And the midterm electorate tends to be more educated, according to Kyle Kondik of the University of Virginia’s Center for Politics. If that trend holds true next year, it suggests a chunk of the core Trump vote will be missing.