It is easy to see why the prediction markets are swinging GOP. If the Democrats want to maintain House control, they had better hope for a dramatic improvement in the economy and the President's approval ratings.
What part of your brain doesn't see the fact that the projection is a disingenuous exercise of propaganda and the DERIVED numbers have NOTHING to do with reality.
from the chart for Reagan/Carter the highest point happens during the first month in the office. this makes sense since people are deluded into thinking that the new president will make a difference. two years later the crowd realizes that the president can make very little difference in their lives so by that point they hate the guy.
in general approval/disapproval should be normalized by the quality of life of the voters at the time.