The Democrats held Congress for 40 years last time, and they have more seats right now than the Republican party has had since the 1920s.
You're only 60-70 years off. 
The GOP had 167 seats in 1990, 18 years ago. They had 302 in the 1920's at one point and bobbed between .
It's actually very likely that, even with a good performance by Obama, the GOP could gain in excess of 20 seats. There have only been three cases when the party out of power didn't gain seats since WWII; two of those (1962 and 2002) were due to redistricting.
Two other factors.
1. The opinion of Congress is in the toilet. That may have been masked somewhat by the Obama victory, but it is still there.
2. A sizable number of those districts were originally drawn to be Republican. PA-4, PA-8, PA-7, and PA-3 are three examples. The demographics can change, but not that much in 8 years in most districts. Some can flip back.
2010-16 may be the key years. I will 49 on the day of the 2012 election, and that might set political patterns for the remainder of my lifetime. That is a scary thought.
Rebounds are the norm. First-term Representatives are the most vulnerable of incumbents, often having won marginal districts under unusual circumstances. Sometimes they are too far from the political norm in their district; sometimes they just aren't up to the job; sometimes they defeated a scandal-plagued predecessor and face someone with no such problem in the following election. Sometimes the new Representative is a family member of the predecessor -- and that, however traditional it may be, might bring someone who lacks something in office. Political fads come and go. Landslides might bring in new Representatives dragging on the coat-tails of a big winner, only for those coattails to drift onto quicksand.
