I always thought it was more of a "redneck" patriotic identity sort of thing, mostly among working-class Anglos (Scots and Scots-Irish, primarily). The distinction has always been presented to me as more sociocultural than ethnic. Basically, they're white-bread, their families have been here forever, damn it, they're Americans.
It does appear to be a sociocultural phenomenon among rural, working-class, conservative whites. There was a thread a while back comparing Kerry/Bush voting with ancestry plurality by county, and an overwhelming percentage of counties with an American ancestry plurality voted for Bush. Perhaps liberals and those in urban areas are more likely to trace ancestry back to Europe and/or identuify with another ethnic/religious/cultural group.
