Update: Clinton now leads in Mississippi 83-16 with 48 percent of precincts reporting, which would be her biggest win of the primary season so far. Her previous best was in neighboring Alabama, at 78-19.

The reason Southern states have been called Hillary Clinton's firewall is not complicated. Clinton leads by a wide margin nationally with older voters, with women and with black voters. Bernie Sanders leads with younger voters, men and white voters. In every state, there are a lot of voters of varying ages and there are lots of men and women.

It is not in every state that there are a lot of black voters. Where there are, Clinton crushes it. We made this graph showing the correlation between black turnout and election results over the weekend.

So when we tell you that preliminary exit poll data reported by CNN showed that 69 percent of the Democratic electorate in Mississippi was black, you can probably reach an obvious conclusion.

If we divvy up the black and white vote in Mississippi by who they backed, this is what results. Clinton got about 9 out of every 10 votes from black voters -- and a majority from whites. All that blue is Clinton. The yellow is Sanders.

Bernie Sanders has needed to make inroads with black voters in order to defeat Clinton. There are some indications that he's gotten support from younger black voters, but younger voters don't turn out as much. Clinton has continually matched or beaten Barack Obama's primary night margins from 2008, a staggering fact. (In Mississippi, she's close but not quite there.)

The fact that the electorate was so heavily black and that black voters were so heavily supportive of Clinton means that Clinton's other advantages were also heightened. Among older voters -- many of whom were black -- Clinton again drove up a huge margin. Six in 10 voters were 45 or older, and 9 in 10 of them went for Clinton, too.

About 6 in 10 voters were women. Eight in 10 backed Clinton. And so on.

We noted over the weekend that Democratic contests were fairly predictable if you knew how many black voters there are. This one was easy.