- The president's party never gains state legislatures in a midterm election, at least as far back as the data go. The best a president can hope for is to lose no state legislatures, as happened with Truman in 1950 and Clinton in 1998.
- The national economy predicts changes in control of state legislatures even better than it predicts changes in the composition of the U.S. House. The R-squared for this bivariate relationship is an impressive .54. It was only .13 for House elections.
- The change in control of state legislatures correlates with the change in House seat shares at .82.
- While the national economy does a good job explaining changes in control of state legislatures during midterm elections, it falls flat during presidential elections.
You can make of this what you want, but this strikes me as good fodder for the debate over whether the economy structures everything that happens in elections or only structures about 95% of what happens in elections.
UPDATE: Tim Storey at NCSL was kind enough to share his data on state legislative seat shares. I've used that to make the following scatterplot:
I used percentages rather than raw numbers since the total number of state legislative seats isn't constant over time (Hawaii and Alaska add theirs in starting in 1962). The relationship between national economic growth and changing state legislative seat shares is statistically significant (p=.003), suggesting that for each percentage point drop in disposable income, the president's party will lose just shy of one percent of its share of state legislative seats. This bivariate regression has an R-squared of .28, only about half of what it was for statehouse turnover above.