X-axis: percent of people reporting English or American ancestry in the 2000 Census (according to Wikipedia)
Y-axis: fraction of votes cast for Obama in 2012 (www.maine.gov/sos/cec/elec/2012/2012prescounty.xls)
This took me about 5 minutes. It could be repeated with more finely-grained data (town/city-level instead of county-level), using more recent ancestry information, and controlling for the presence of non-whites. But I don't anticipate the trend reversing.