Georgia’s 6th is located just above the small oval-shaped district in the north of the state, in the middle of three districts. (The Washington Post)

One of the ways that partisanship overlaps with American society is along racial lines. The Republican Party is mostly white, as Gallup polling makes clear; the Democratic Party is more diverse.

The vast majority of black Americans vote Democratic.

It’s fair to wonder how that racial split is affected by — or affects — political polarization, but it has a tangible effect on American politics. The Democratic primaries in 2016 were heavily shaped by Sen. Bernie Sanders’s inability to persuade black Democrats to support his candidacy. Southern states with large black populations gave Hillary Clinton a substantial lead that she never lost.

More broadly, the overlap of race and politics can be seen clearly in the House. On average, seats held by Democrats are about half white (meaning, in this case, white non-Hispanic) and about half nonwhite. Seats held by Republicans are about three-quarters white.

There’s a lot in that chart, so it’s worth clicking on it to make it larger. (Note, by the way, that Sanders’s former House seat — the at-large district in Vermont — is the whitest Democratic seat in the country.)

One thing to note is where the 6th District in Georgia falls — right at the midpoint between the average Democratic and average Republican seat. This is a bit misleading, because white Americans tend to vote more heavily than do nonwhites. (In 2016, 60 percent of voters in Georgia were white, while only 54 percent of the population is white non-Hispanic.) But the composition of the population offers some insight into why the district is currently contested.

When we look at the 2016 results by congressional district (using data compiled by Daily Kos) and overlay the density of the white population in each district, there’s a clear pattern of whiter districts more heavily favoring Donald Trump. Right at the center of that cloud? Georgia’s 6th District.

Eight Democratic-held House districts (4 percent of them) backed Trump more heavily than the 6th District, and 26 Republican districts (10 percent) were more heavily supportive of Hillary Clinton.

The fight for the 6th District in Georgia is defined by far more than the racial composition of the constituents, of course, just as the partisan split nationally has broader causes and effects. But we will note, too, that the proportion of the population that’s white in Kansas’s 4th Congressional District is 74.6 percent, just a bit above average for the Republicans and well above average for the Democrats.

That’s the seat that looked as if it might be close ahead of a special election last week. The Republicans held it by a narrower-than-expected-but-still-big-enough margin.