How the black vote carried Andrew Gillum to victory

As results of Tuesday night's primary election for governor rolled in, one thing became clear: bigger, blacker counties wanted Andrew Gillum.

Looking at a map of the county's margins -- showing both who won every county, and how many votes they stacked up over their opponent, one county stands out: Duval.

The Jacksonville-area jurisdiction isn't the most populous in Florida, but it's where Gillum built up his widest margin over former-frontrunner U.S. Rep. Gwen Graham. In a race with five serious candidates, he took more than 51 percent of the vote, earning more than 40,000 votes to Graham's 18,000.

That 22,000-vote difference was huge for the Tallahassee mayor's win. Erase Duval, and Gillum's lead over Graham would be cut in half.

If you only look at registered Democrats (the only people eligible to vote in the primary), Duval also stands out as the largest, by far, of the four Florida counties with a majority black Democratic voting population. Some 55 percent of Democrats there are black.

It was the clearest example of a trend: Gillum won a few enormous counties with lots of black voters. The smaller counties -- with whiter populations -- that Graham won couldn't match up.

All along, Gillum's campaign insisted pundits shouldn't count out the black turnout. Tuesday, it was proven right.

All across the state, the share of black members of a county's registered Democrats tracked exactly with how well Gillum did. Counties where large shares of Democrats are black, like Duval, Escambia and Broward, overwhelmingly supported the mayor.

Of the 21 counties with the highest share of black voters, Gillum won 16. Of the state's other 46, he won just 2.

Graham took many of those smaller counties with higher shares of white Democrats. Her biggest wins -- Pinellas, Sarasota and Volusia -- were in purple Central Florida. Unfortunately for her, those places just don't have as many voters as Duval or South Florida.

Her loss was a repudiation of a golden rule in Florida politics -- that winning the vaunted Interstate-4 corridor meant winning the entire state.

She unquestionably won Tampa Bay, finishing first in Pinellas, Pasco, Hernando, Manatee and Sarasota, with a close second in Hillsborough.

But those modest gains were outdone by South Florida, where she was trounced.

Gillum's huge wins in Broward and Miami-Dade were assisted by Philip Levine's strong showings there. In both counties, the former Miami Beach mayor finished second, pushing Graham down to the third spot. Gillum scored twice as many votes as Graham did in those counties.

Republican hopeful Adam Putnam similarly won lots of small and medium-sized counties in Central and North Florida but was outdone by hordes of Ron DeSantis voters at the state's northern and southern ends.

Looking forward, the map changes for both Gillum and DeSantis as they seek voters from each other's party and those affiliated with no party at all.

For example, Duval's Democrats may be mostly black, but the county as a whole is nearly two-thirds white. With a ratio like that, what remainder of Duval can Gillum win?