Originally Posted by Dino 6 Rings
It actually happens almost everytime a new party takes the whitehouse. Heck even when Reagan won the Whitehouse, the Republicans lost seats in the House and Congress. The Single Party rule really turns people off and since the white house (regardless of who is in) can not satisfy all the people all the time, when the election comes around mid term, the people vote against the white house party on a regular basis.
Check and Balance.
Even bigger reason for midterm losses is what political scientists call "surge and decline". When a party wins the White House, it is, by definition, a good year for that party. Which also means that party wins seats it wouldn't normally win. Two years later there's a regression to the mean and they lose those seats.
Which is also one of the reasons it didn't happen in 1998 and 2002. Clinton won by a plurality and Bush lost the popular vote, so there wasn't a high to come down from two years later. Impeachment and the war on Islamofascism, respectively, pushed the midterms even further in the in-party's direction.