Using history as a guide, Charlie Cook argues that the 2014 election will be bad for Democrats:
The 2014 vote is what’s known as a "six-year itch" election, with the party holding the White House usually losing a substantial number of House and Senate seats in the sixth year of its tenure. There are a variety of reasons, but at that midway point in a party’s second four years in the White House, the "in" party tends to lose energy and focus. Party leaders run out of ideas, and the "first team" in terms of personnel—the people who were there when the president took office—have often bailed out, and the second or third team is sometimes not as good. Voters tend to grow weary and to look for something different.
In the six "six-year itch" elections since World War II, the party in the White House has averaged a 29-seat loss in the House and a six-seat (actually 5.6) loss in the Senate.