Businessman David Perdue won the runoff in Georgia's Republican Senate primary on Tuesday, beating Rep. Jack Kingston by about 1.6 percentage points. In the primary, held in late May, Perdue beat Kingston by almost five percentage points -- meaning that Kingston picked up more of the votes earned by candidates eliminated from the race than Perdue. Except: Perhaps not.

First, let's look at how the vote broke down by county.

Vote margin
Percentage margin

As you can see in that first map, Perdue won largely thanks to heftier vote totals around Atlanta, in the northwestern part of the state. Kingston did the best around Savannah, which is his political base. And, in fact, as the second map shows, Kingston's margin of victory in the southeastern part of the state was much larger than Perdue's in the northwest. But there are a lot more votes in the counties surrounding Atlanta than where Kingston did best.

But the really interesting story is turnout. In the primary, nearly 604,000 people cast votes. In the runoff, that dropped to 482,000. And the areas that saw the biggest drops were the areas with the largest populations, unsurprisingly. But that meant areas that backed Perdue.

Instead of Kingston picking up votes from supporters of eliminated candidates, many of those supporters didn't bother voting in the runoff. There's a correlation between how many primary votes went to candidates not named Kingston or Perdue in the primary and how much turnout dropped in the runoff.

Here's where Kingston and Perdue did better in the runoff than they did in the primary. Kingston gained a lot of ground in the areas where Perdue did the best; Perdue did better mostly in places without many voters.

Change in Perdue support
Change in Kingston support

The natural next question then, is: what does this mean for November?

Well, there's reason for some trepidation on the part of Perdue. The map below shows which counties have the highest density of African-American voters (as of last November). There's a sweep across the middle of the state (which, amazingly, is thanks in part to an ancient coastline), but also significant pockets in the northwest.

If Kingston voters decide to stay home in larger numbers in November -- as the primary candidate supporters did for the runoff -- and if black voters vote more heavily Democratic in the places where Perdue did well, that's a troublesome combination.