Racial demographics were a major factor in the partisan shift of these counties, but it cannot be denied that there was a substantial number of voters-moderate suburbanites who voted for Romney and/or George W. Bush, who moved towards the Democrats in recent years. In Orange County, and in San Diego County, this is certainly true. Even in Cobb and Gwinnett, there were many Romney-supporting suburbanites who shifted Democratic in 2016 and particularly last year. Henrico and Prince William are counties full of moderate suburbanites who have soured on Republicans in recent years.
-Cobb and Gwinnett went from 90% white in the early 90s to around 45% white now. In GA, Republicans get around 75% of the white vote, but due to the increase in minorities, the state has gotten tighter in recent years. So GA's transformation is purely due to racial demographics. VA is different as the primary catalyst is the influx of college-educated white liberals into northern VA.
-OC and SD are a functin of both racial demographics and white suburbanites, but the correlation between GOP vote share and % of whites, is pretty hard to deny.
-There's a very good reason why Democrats want mass immigration and citizenship for illegal aliens. It's about winning elections.