Larry Sabato asks: Can recent history suggest anything about the 2006 Senate results? On average, the president’s party has lost three Senate seats in each of the last 14 elections (from 1950 to 2002); but this includes all midterm elections, both the first midterm election of a presidency and the second one in the sixth year of the two-term presidency.
There have been six "sixth year itch" elections in the post WWII era (1950, 1958, 1966, 1974, 1986, and 1998): The average loss for the White House has has been 6 Senate seats. And, this is the important point: "Never in modern times has a president been able to add Senate seats in the dreaded sixth-year election," for more details.
At most five or six Senate seats out of 33 will be open, without an incumbent, making party turnover harder, though not impossible, writes Sabato. There 18 Demo seats up and only 15 GOP. Tough for the Democrats to take the Senate back. And, IMHO, if the GOP picks up even one Senate seat, there is a realignment (Sabato doesn’t note this).