Recent weeks have seen regular openings of new Obama field offices across Virginia (there are now 15), and the dozens of paid staffers working out of the campaign headquarters in Richmond and elsewhere around the state have become increasingly visible.

In what both sides regard as one of the election’s three or four most critical swing states, Obama has opened up a slim yet significant three-point lead in the latest RCP average of Virginia polls. Though he shows strength in other regions of the state, the president largely has the expansive D.C. suburbs to thank for that advantage.

In Fairfax, the Old Dominion’s most populous county, Obama bested John McCain by 61 percent to 39 percent in his seven-point Virginia victory in 2008. While he may not have to win the county by that wide a margin this time around, he is counting on Northern Virginia’s increasing diversity and its large federal workforce to provide a critical edge once again.

“The economic influence of the federal government is probably outsized here,” said Rep. Gerry Connolly, a Democrat who represents Virginia’s suburban 11th District, where residents are among the wealthiest in the nation. “We [also] look a lot more like the face of the country as a whole than ever before, and what’s interesting about that is it tends to favor Democratic candidates.”