The main reason is that the northern Virginia suburbs of Washington are growing faster than other areas of the state.
Northern Virginia and the rest of the state voted very differently in the election last Tuesday. I reviewed preliminary, uncertified vote totals in the U.S. Senate race from these 9 northern Virginia jurisdictions — Arlington, Fairfax, Loudoun and Prince William counties, and the independent cities of Alexandria, Manassas, Manassas Park, Fairfax City and Falls Church.
This is what I found:
Northern Virginia: Democrat Jim Webb won by a margin of roughly 122,000 votes.There was also a noticeable gap in how the two geographic groups voted on Amendment 1, the constitutional ban on same-sex marriage. In these same northern Virginia jurisdictions, the amendment banning same-sex marriage lost, but the amendment won more than 60% of the vote downstate.
Rest of the State: Republican Sen. George Allen won by roughly 113,000 votes.