Recent articles such as this discuss that his mother, while born in Kansas, spent her middle and high school years near Seattle and much more liberal than she was portrayed.
http://www.chicagotribune.com/news/politics/chi-0703270151mar27,0,5157609.storyThere are several MSM articles like this to support the basis of the Repub attacks.
Now, to see how it is being used, try searching on "obama mercer island marxist" (or marxist->marx).
Then search on Obama's apparent mentor in Hawaii, Frank Marshall Davis, and add him to the mix.
Do you want more links or is this enough?