Republicans are normally the majority when the population density is below 1,000 people per square mile. Big gov lovers take control above that. Housing builders can turn any Republican area Democrat simply by building like this:
In 1990, 15% of the population of Fairfax County was foreign-born. Today, it is 30%. Couple that with the expansion of the federal government and the Beltway bandits it attracts, you have the formula for Dem domination. VA will soon be a solid blue state. North Carolina is not far behind.