Here’s an interesting relationship. The graph shows the percentage Republicans in the states Lower Legislatures versus the Upper Legislatures.
It would appear that if you’re a Republican, you have the best chances of winning an election in the lower house in states where the upper house is split. You will have a hard time in states where the upper house is either mostly Republican or Democratic.
The data for this spreadsheet has been uploaded to this website’s wiki: Republican and Democratic Economic Data, Feb 2011