No single factor explains any election. First-term presidents almost always face a backlash from a ginned-up opposition, and hardly a day has passed over the last two years in which Trump has not done something to gin up his opposition. But for the almost infinite list of reasons voters broke the Republican hammerlock on the House, health care has been the dominant theme. It is their most visceral and undeniable betrayal of their promises to the voters. Attempting to repeal Obamacare probably cost Republicans the House.