I looked at all national elections since 1946, paying particular attention to the midterm elections. This is of particular interest because of what the Dems might be facing in the Fall elections. Based upon my viewing of history
1. What is the average number of seats that the President's party loses in a midterm election.
Starting from 1946 24.4
Starting from 1970 19.1
Starting from 1990 15.8
The lesson is that more recently the seats that are lost diminishes significantly from what it was at the middle part of the last century. In fact the only two times that the President's party gained seats since 1946 were in the recent midterms (1998, 2002).
2. The five worst cases of the President's party losing seats in the midterm is below
Year Result Reason
1946 Reps +55 Labor unrest, Truman not demobilizing fast enough
1994 Reps +54 Dems fail to pass health care, Reps contract with America
1974 Dems +49 Watergate
1958 Dems +49 Eisenhower blamed for Russian space suceess, bad economy
1966 Reps +47 Unrest about Vietnam War, rioting in streets
You can't find any instance where passing something caused a wave election. Generally what causes problems for the President's party in midterm elections is: economy, corruption, inertia.
Interestingly in only 3 of those 5 wave elections (1974, 1958, 1966), was predictive of the following Presidential election. And each of those Presidential elections was very close. And the 1946 midterm was not AT ALL predictive of the 1948 election, where not only did Truman win but Dems won 75 additional Congressional seats. Truman campaigned against a "do nothing" Congress.
3. The most important/controversial piece of legislation during this post World War 2 period was the 1964 Civil Rights Act.
This was filibustered (real filibusters not like the phony kind they have today) for 57 days. It finally passed on July 2, 1964. Democrats started that Congress with 259 seats. What hell did they pay for going against their own constituency and doing the right thing?
The Dems WON 36 additional seats, bringing their total to 295 Congressional seats, which is the high water mark for either party since 1936.
Certainly some of the reason the Reps lost was because they nominated the polarizing Barry Goldwater and Lyndon Johnson won in a landslide.
However I might argue that one party's success causes the other party to do crazy things. (similar to what we are seeing today)
Also winning a Presidential landslide doesn't always win you many seats. Republicans won landslides in 1972 (Nixon) and won just 12 House seats and in 1984 (Reagan) and won 16 House seats. And in both cases they had many more seat possibilities than the Dems did in 1964.
4. Based on history, a number of reps will lose their seats during the Obama Presidency.
History says that over time the President's party loses seats. While you can't fight the political undertow at least Dems will have voted and passed significant legislation.
President Started Ended Change
Eisnhower 221 174 47
Kennedy/Johnson 263 243 20
Nixon/Ford 192 143 49
Carter 292 242 50
Reagan 192 175 17
Bush - 41 175 176 -1
Clinton 258 212 46
Bush - 43 221 178 43
It is interesting that Dems (except Carter) started with about the same number of house seats. Of course, the Dems of today are considerably more liberal than the Dems in earlier times. It does help you admire the good work that LBJ did passing legislation. Of course, the Reps were much more cooperative to work with.
2010 could be a midterm for the history books. Fairly popular President, strong (comparatively) legislative record, dysfunctional oppostion.