In the 2010 United States midterm elections, the Democrats lost a net of 680 state legislative seats, which were gained by the Republicans. This manifested itself in 21 chambers which the Democrats lost control of and 19 which the Republicans gained control of. Have there been any previous instances where such a large shift in state-level legislative power took place? As there used to be fewer states, this may manifest itself in smaller numbers, but equal or larger proportions.
Federal Election Results
According to The American Presidency Project, the 2010 midterm election holds 2nd place in having flipped the most U.S. House and U.S. Senate seats.
By position, the top five midterm elections which have resulted in the most U.S. Congress (House and Senate) seats lost / flipped are as follows:
- 1938 (Franklin D. Roosevelt), House Seats Lost: -71; Senate Seats Lost: -6
- 2010 (Barack H. Obama), House Seats Lost: -63; Senate Seats Lost: -6
- 1942 (Franklin D. Roosevelt), House Seats Lost: -55; Senate Seats Lost: -9
- 1994 (William J. Clinton), House Seats Lost: -52; Senate Seats Lost: -8
- 1958 (Dwight D. Eisenhower), House Seats Lost: -48; Senate Seats Lost: -13
Close behind at #6 was the post-Watergate midterm election of 1974 (Gerald R. Ford having replaced Richard M. Nixon), House Seats Lost: -48; Senate Seats Lost: -5.
State Legislative Election Results
According to Wikipedia's article on United States Elections, 2010, the 2010 elections hold 1st Place for state legislative control changes (680 flipped by Republicans), with 2nd Place honors (dishonors?) going to the post-Watergate 1974 United States Elections where the Democrats flipped 628 state house seats.
Gubernatorial Election Results
I have only been able to find compiled data going back to 1990 for Governor's races (Source for data below: Dave Leip's Atlas of U.S. Presidential Elections - includes all Governors' race results).
(Note: I did find an interesting online source with detailed federal and local election results from 1787 - 1825 (A New Nation Votes) but the data is not presented in an easily comparable way and would require days or weeks to compile and compare.)
But here below, using Dave Leip's data, are the midterm Gubernatorial seat-flip results from 1990 to present (2014 being the most recent until 2018 results are in) sorted by largest overall net change to smallest, showing net gain/loss for (R)epublicans, (D)emocrats, and (I)ndependents / Reform / Other.
(Note: net change does not balance to 0 in all cases):
- 1994: (William J. Clinton), (R) +10, (D) -10, (I) 0
- 2006: (George W. Bush), (R) -6, (D) +6, (I) +1
- 2010: (Barack H. Obama), (R) +5, (D) -6, (I) 0
- 2002: (George W. Bush), (R) -1, (D) +3, (I) -2
- 2014: (Barack H. Obama), (R) +2, (D) -2, (I) 0
- 1990: (George H.W. Bush), (R) -1, (D) -1, (I) +1
- 1998: (William J. Clinton), (R) -1, (D) 0, (I) +1
The total net change for both of these elections exceeds that of the 1938 elections (a loss of 71 in House and 6 in Senate for 77 total).
A large scale collection of county-level results, state by state from 1824 to 1968, has been collected by University of Michigan. However its organization as separate files by state and era, makes analysis a project beyond a reasonable scope for one answer on this site.