The election held in the 6th year of a president’s tenure in which the party holding the White House historically loses a substantial number of House and Senate seats.
The Atlantic: “For decades political analysts have been intrigued by an ironclad pattern in American politics: the President’s party loses seats in the off-year election that follows his White House triumph–a phenomenon that has occurred in every off-year election save one since the Civil War. Since the Second World War, off-year losses for the President’s party in the House have averaged fifteen seats in the second year and forty-eight in the sixth; in the Senate the average losses are zero in the second year and seven in the sixth.”
Charlie Cook: “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.”