This map shows the relative strength of the two parties, based on an analysis of votes in key races in 2008 and 2010:
Obama v. McCain for president
Gregoire v. Rossi for governor (2008)
Murray v. Rossi for Senate (2010)
McMorris Rodgers v. Romeyn (2010)
Mager v. French (2010)
There were several shifts in precincts between the two elections, and results were adjusted to reflect those changes.
For a closer look at the map, click on the PDF file.