The “six-year itch” is the election held in the sixth year of a president’s tenure.
In this election, the party holding the White House historically loses a substantial number of seats in the House and Senate.
Overall, the six-year itch phenomena is an extension of the “midterm effect” where a president’s party almost always loses seats in midterm elections.
The seats lost during a president’s second midterm have tended to be even more pronounced than during their first midterm.
And since the president is not on the ballot, there is no coattail effect.
From 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 adds:
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.