Sometimes looking at vote-share by percentage doesn't tell the whole story. Raw vote totals show us how crucial turnout is to winning the House.
In the chart below, vote totals are shown for candidates for the U.S. House by party from the past eight general elections. Not surprisingly, turnout is much higher in Presidential Election cycles than it is in midterm cycles.
What I find interesting is that 2018 sticks out as an anomaly - 2018 Democratic turnout looks much more like a Presidential year than a midterm. Republican turnout is also high, but not nearly as much so.