Real Clear Politics has constructed an election index that attempts to quantify Republican and Democrat party strength based on five values. I think it's a valuable tool for analyzing the disparate numbers.
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.
This gives us five metrics, all of which run on a scale from 0 to 100. Adding them together gives us a scale from 0 to 500. We then subtract 250 from the total. All this does is assign a score of zero to a situation where the parties are evenly matched, rather than 250. A positive score then means that the Republican Party is stronger while a negative score means the Democratic Party is stronger.