The dirty little secret is a lot of democrats are liberal on social issues but much more moderate on economic issues. They have no more desire to see Bernie Sanders raise their taxes or crash the stock market than we do. I’m guessing if Bernie secures the nomination we are going to start hearing some whispers from the more moderate wing of the democrat party that...perhaps...Trump getting four more years isn’t the worst thing that could happen and democrats should concentrate instead on holding the House and taking the Senate.
The problem with that strategy is turnout.
Folks who are not excited about their presidential candidate don’t vote in high enough numbers to turn any other reasonable close race. That is the dirty little secret about “coat-tails”. The non-voters of the losing party are just as important as the additional voters of the winning party.
2016 was the classic example, where blacks in MI and WI failed to turn out for Hillary, which in turn doomed the down ticket Democrats.