no expert hat here, more of a question.
Of all those times both the house and Senate flipped, what was the most Senate seats?
It just seem like a lot to gain 8, 9, 10 or 11 in one cycle.
The Republicans gained 56 seats in the House and 13 in the Senate when they took control of both chambers in 1947.
The Democrats gained 97 House seats and 12 Senate seats when they took control of both chambers in 1933. (Republicans lost 101 in that Great Depression election that swept FDR into power).
The biggest Republican gain was a 120 seat gain in 1895. They added six Senate seats to control the chamber. (This shift was induced by the Panic of 1893).