Looking back on Waves standards and the 2018 elections

Last year, Ballotpedia’s Wave Elections report used historical data to produce a methodology for determining the number of seats needed for an election to qualify as a wave. In the report, Ballotpedia defined a wave election as the 20 percent of elections in the last 100 years with the largest seat swings against the president’s party.
The report then looked at the number of seats Democrats needed to win in House, Senate, gubernatorial, and state legislative elections for 2018 to be considered a wave election—48 House seats, seven Senate seats, seven gubernatorial seats, and 494 state legislative seats.
By that definition, one election type met wave standards – the gubernatorial elections, where the Democratic Party gained seven seats. Elsewhere, Democrats gained 41 or 42 House seats (depending on the results of the NC-9 special election), lost one Senate seat, and gained 349 state legislative seats.
How does Democrats’ 2018 performance compare to the other 50 elections from 1918 to 2016? Ranking the performance of the out-of-power party by the number of seats gained, we found:
  • Democrats had the 13th strongest performance in House elections
  • Democrats had the 29th strongest performance in Senate elections (tied with seven others)
  • Democrats had the 10th strongest performance in gubernatorial elections (tied with two others)
  • Democrats had the 15th strongest performance in state legislative elections
Within a more modern subset of election years, from 1946 to 2016, the wave election threshold was lower: 30 House seats, six Senate seats, five gubernatorial seats, and 344 state legislative seats. By this measure, House, gubernatorial, and state legislative elections were all waves.
The difference in the thresholds occurs because several of the largest historical waves (1920, 1922, 1930, 1932, and 1938) were before 1946. At times, political scientists separate the study of American elections into pre-1945 and post-1945 periods to account for the social change and political realignment the nation went through during the Great Depression and World War II.