For McAuliffe, his victory was not built on the raw number of counties (and cities) he was able to turn from red to blue but rather the gains he made in a handful of population-rich counties clumped in northern Virginia. McAuliffe took 59 percent in Fairfax County with a margin of roughly 66,000 votes. Democratic nominee Creigh Deeds lost the county in 2009. In neighboring Prince William County, McAuliffe won with 52 percent, a margin of roughly 8,000 votes. Four years earlier, McDonnell won 59 percent in Prince William -- wracking up a 13,000 vote margin. In Loudoun County, a fast growing western Washington exurb, McAuliffe won by four points and 4,000 votes; McDonnell had carried the county with 61 percent and by almost 15,000 votes in 2009.
Add it all up and here's what you get. McAuliffe left Loudoun, Fairfax and Prince William with a 78,000 vote edge over Cuccinelli; four years earlier, McDonnell left that trio of NoVa counties with a 32,000 vote lead over Deeds. When you consider that McAuliffe's statewide margin was only 55,000 votes (or so) over Cuccinelli, you begin to grasp just how critically important the inner and out suburbs of Washington have become to the fate of candidates statewide in Virginia.
(Worth noting: The two counties-- Bath and Alleghany -- that went from Democratic in 2009 to Republican in 2013 is explained by a bit of regional favoritism; Deeds is from Bath County and represents Bath and Alleghany in his state Senate district.)