In 2008 in Virgina, President Obama won 53.2% of the two-party vote share (i.e. Obama votes / (Obama votes + McCain votes)) . In 2012, he won 51.5% of the two-party vote share. Where did the percentage point drop (1.7%) come from?
Virginia has 134 counties and cities. In 21 of those unit, Obama saw an absolute increase in vote-share percentage in 2012, and in an additional 42 units, his vote-share percentage decreased by less than the 1.7 percentage point decrease, state-wide total. In 71 of the counties/cities, Obama 2012 two-party vote share percentage saw an absolute decrease, including 6 counties/cities where he saw a 5-10 percentage point decrease, and 6 others were he saw a decrease of more than 10 percentage points.
Below is the scatterplot of Obama county-level vote share in 2008 vs. 2012, with a 45 degree line drawn in. Points below the line indicate Obama did worse in 2012 than 2008 in that county; points above the line indicate he did better.
A few observations:
First, note that the places where Obama saw large decreases from 2008 to 2012 were almost exclusively places where he had already done badly in 2008. In fact, the only two places Obama won in 2008 that he lost more than 4.5 percentage points were Lexington City and Westmoreland.
Second, note that the general trend roughly reflects the outlier trend. The swing was not uniform. Obama did somewhat worse against Romney in McCain ’08 counties, but somewhat less worse in Obama ’08 counties. The places where he did better in 2012 were almost all places he won in 2008.
Finally, note that the outliers are quite small counties; Obama’s 0.8% worse showing in Fairfax County — where there were over 400,000 voters — maters more in an absolute sense than the 10% worse showing in the outlier counties, which combined have only about 130,000 voters.