When William E. Davis was growing up here in DeSoto County, just across the state line from Memphis, there were more than 300 dairy farms, and he was raised on one of them.
“Now there are zero,” said Mr. Davis, 66, who is known as Sluggo, the chancery court clerk in a county that has been transformed into a booming suburb of over 168,000 residents.
About 800 miles to the east, the same kind of sweeping changes have taken hold in the sprawling suburbs around Richmond, Va., where woods and farmland have been turned into gleaming new subdivisions with names meant to evoke the state’s colonial past.
In both states, the growth fueled by a migration of newcomers from other parts of the country and even abroad is bringing nationalized politics to races further down the ballot. It was these new arrivals, more than any other voters, who most crucially rejected two influential Republican incumbents — the House majority leader, Representative Eric Cantor of Virginia, and Senator Thad Cochran of Mississippi — in primaries this month, upending long-held assumptions about the appeal of traditional levers of power.
In the newly built communities of DeSoto County in Mississippi, and the fast-growing precincts in such metropolitan Richmond counties as Henrico, Hanover and Chesterfield — what could be called the Chick-fil-A belt — the conservative challengers to the two incumbents led by overwhelming margins.
State Senator Chris McDaniel thrashed Mr. Cochran by 36 percentage points in DeSoto County, the state’s third-largest, a key factor in his edging the incumbent by fewer than 2,000 votes statewide and forcing Mr. Cochran into a runoff. Mr. Cantor lost the suburban population centers of his district by double digits en route to being unseated by David Brat, a conservative economics professor and himself a Virginia transplant.
For all the talk about how partisan polarization is overwhelming Washington, there is another powerful, overlapping force at play: Voters who are not deeply rooted increasingly view politics through a generic national lens.