The Rhythm of Senate Elections

From reading media reports of the 2014 election results you might believe the nation has experienced a political change of cataclysmic proportions. Republicans won 23 of the 36 states where a senatorial election was held, enough to give them control of the Senate for the next two years. Yet we need look back only to another strong Republican year, 2010, to see nearly identical results. In that year the Republicans took 24 of the 37 states where elections were held.

Historically the parties’ shares of Senate elections have swung back and forth quite substantially with the last decade appearing unusually unstable.  Here are the results for Senate elections back to 1936:



The Democrats reached their peak in the 1964 Johnson landslide, though this election merely confirmed the Democrats dominance of the Senate “class” elected six years during the 1958 Eisenhower recession.  Republicans have won about two-thirds of the seats in half-a-dozen elections over the same period of time.  The 2014 result is quite similar to the Republican margins in 2010, 2002, 1980, 1952, and 1946.

If you look carefully at this graph, you’ll see a certain rhythm in these results, one created by the six-year length of a Senatorial term and the power of incumbency.  In fact, if we slide the graph forward six years and superimpose the results, we get this:


Now we see how the partisan split in a Senate “class” helps explain the variation from election to election.  Because incumbents have an advantage when it comes time for re-election, the partisan composition of a Senate class tends to repeat at six-year intervals.  The Republican edge in 1946 was replicated six years later when Eisenhower won the White House. Six years after that a major political shift occurs.  The largely Republican class of 1946 and 1952 was replaced with a largely Democratic class during the Democrats sweep of the 1958 off-year elections. The Republicans’ victories in 1980 constituted a similar shift for their party.

Of course the partisanship of the class facing re-election just sets the stage on which each year’s set of electoral forces plays out.  We would expect that factors like broader trends in partisanship and the state of the economy might also influence the outcome of Senate elections.  I turn to those influences in the next article.