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To get the percentage we computed seats lost / initial seats for both chambers to get individual seats lost percentages. We then average the percentages together to get a single number, essentially weighting the seats (Senate vs. House) by their relative individual significance.
Congressional elections are often counter-cyclical, and a recent question that has arisen is how out of line the Republican gain in the legislature and at the state level with historical norms.
The theory is that while a presidential election may have coattails, they are more than offset but subsequent mid-terms, and higher mobilization of the opposition in the end of the president's term. So we will be measuring the difference between the number of seats held in January 2001 vs. number of seats held in January 2009. It is worth noting that this does not give us the greatest difference, since early vacancies generally swing towards the President's
On opening day in 1961 the split was 65-35 (favoring Democrats). On opening day in 1969 the Senate was split 57 to 43, favoring the Democrats. It represents a loss of 8 seats, or 12%.
In 1961 the Democrats dominated 264 to 173. In 1969 the Dems still had a large advantage: 243 to 192, but had lost 21 seats for an overall loss of 9%.
Data -- every year of the Senate
Party standings on the opening day of the 91st Congress
57 Democratic Senators
43 Republican Senators
TOTAL members: 100
TOTAL members: 435