Coming into Tuesday's runoff election in Mississippi, polling suggested that incumbent Sen. Thad Cochran was at a major disadvantage. As we noted, there was a big question mark hanging over any predictions because it wasn't clear what turnout would look like.In part, that was because the Cochran campaign was explicitly targeting black voters to come out to vote, despite those voters usually voting Democratic.

Now, of course, we know what happened.

Counties with a higher black population voted more heavily.

In Mississippi's 24 counties with a majority black population, turnout increased an average of 40 percent over the primary. In the other 58 counties, the increase was only 16 percent.

You can see the overlap on the maps below.

Black population

Turnout increase

Or, if you prefer graphs, here's black population percentage plotted against turnout increase.

Cochran did better in counties with increased turnout.

In the places where turnout increased, Cochran tended to do better. It's not a super strong correlation, but it exists.

Percent support for Cochran

Support v2

But that increased turnout only really mattered in a handful of counties.

Most of those counties, however, didn't deliver many votes for Cochran.

Total votes for Cochran

At the end of the day, Cochran's margin of victory can be attributed to a shift in two counties: Jones (McDaniel's base) and Hinds (Cochran's base). They're the dark purple and near-white counties on the map at left below. Hinds is easily spotted in the map at right, which shows the change in vote margin for Cochran between the two races.

Vote margin

Change in vote margin

In the primary, Cochran won Hinds County by 5,000 votes. His opponent, Chris McDaniel, won Jones County by 9,209 votes.

In the run-off, Cochran won Hinds by 10,965. McDaniel won Jones by 9,876, despite turnout increasing nearly 19 percent over the primary. Hinds County, which is nearly 70 percent black according to the most recent Census estimates, saw turnout increase almost 50 percent.

McDaniel gained more votes in several other counties, but it wasn't enough. Cochran won on turnout, and won the race.