Bill Bishop, who provides
the above map, compares county-level voting from 2008 to 2012 and finds that "[o]nly 208 counties changed allegiance in 2012 out of more than 3,100 counties that cast votes." He provides some perspective:
Statistician Robert Cushing checked all the presidential elections in the last 100 years and found that, on average, 24 percent of all counties switch parties from one election to the next. The 208 counties that changed from 2008 to 2012 amounted to less than seven percent of all counties. That is the fewest flippers of any election in the last century.