Two things to keep in mind about the midterm elections:
- It wasn't just a win for the Democrats, it was a landslide. According to Wikipedia, the Democratic party captured 57.7% of the House vote. By contrast, no presidential candidate in the last 20 years managed to achieve this.
- As a consequence of gerrymanding, the actual results of the election are heavily skewed towards Republicans. Democrats have so far won 53.3% of the seats in the House, with a small number of seats still undecided. Compare that with 57.7%, and you have about a 4% edge for Republicans built into the system. Even the supposedly most democratic element of the American system of government - direct election to the House of Representatives - is not very democratic at all.