At Real Clear Politics, Sean Trende and David Byler explicate their index of party strength. To compute their index, Trend and Byler calculate five components, which they weight equally:
Our index is the sum of five parts: presidential performance, House performance, Senate performance, gubernatorial performance and state legislative performance. The first is measured by the party’s performance in the previous presidential popular vote (NB: In this, and all other measurements, third parties are excluded).
House performance is the average of the popular vote for the House and the average of the share of the House won by the party. This helps mitigate the effects of gerrymandering. Senate performance is the share of the Senate held by the party.
Gubernatorial performance is the party’s share of governorships (again, with third party candidates excluded). We do not weight for population, for reasons explored further below. For state legislatures, we average four numbers: the share of state Houses and state Senates held by each party along with the share of state House seats and state Senate seats held by each party.
So this index is broad-based and represents a comprehensive view of the major parties’ relative strength. Currently, the Republicans are at their highest level since 1928. This chart shows the index from 1928 to the present. Click to enlarge:
What jumps out at me is that holding the presidency consistently leads to a drop in a party’s overall strength. Republican strength declines during the chart’s red zones–Republican presidencies–and increases in the chart’s blow zones–Democratic presidencies. The closest thing to an exception is the Reagan-Bush years, when the Republican Party generally came close to holding its own.
So the fact that the Obama administration has been good for the GOP shouldn’t be surprising; it is consistent with the history of both parties since 1928. Voters seem to prefer, consciously or not, to balance the power of the White House by promoting the opposing party at all other levels. That is, perhaps, the silver lining for whichever party doesn’t control the presidency.