Let’s start with the basic fact that Trump won just 45.9 percent of the vote in 2016. That doesn’t make his victory any less legitimate — winning (the Electoral College) with less than a majority is still winning — but Trump has a smaller base than every president elected since 1972, except for Bill Clinton in 1992. Trump voters are not a majority.
More importantly for the sake of 2018, they don’t represent the majority of voters in the majority of congressional districts. Trump won more than 50 percent in 205 of 435 districts. If House Republicans won every district where Trump won a majority in 2016 but lost every other one, Democrats would control 230 seats. Among seats won by a Republican in 2016, Trump fell short of a majority in 40 districts. Democrats need to win only 24 of those to win control of the House.
Of course, Democrats are unlikely to run the table in districts where Trump got less than a majority. He still won a plurality in 25 of those districts. And Democratic candidates probably won’t win every voter who cast a ballot for Hillary Clinton or a third-party candidate in 2016. The larger point is just that Republicans need more than Trump voters to hold onto the House.