Consider new Monmouth University polling on two of those Super Tuesday states: Alabama and Oklahoma. In Oklahoma, Sanders leads by 5 points. In Alabama, Clinton leads by 48. The difference? In 2008, 18 percent of the Democratic primary electorate in Oklahoma was non-white. In Alabama that year, 56 percent was.

Indeed, we can compare the percentage of the non-white vote to either results (from the first four states) or Super Tuesday polling, and the correlation is fairly obvious. (See footer for links to all of the polls.)

The dots move up and to the right. The greater the percentage of the electorate that's not white, the wider the margin of support for Clinton. (A similar thing happened in 2012, with Mitt Romney's primary performance correlating with the percent of the state that was evangelical.)

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"The best chance for Sanders seems to be in places with largely white Democratic electorates," Monmouth pollster Patrick Murray wrote about his survey, with no small amount of understatement. Another way of saying that is that the best chance for Clinton seems to be in places with largely black voters.

If we compare just the black vote (in the 2008 primary or in the first four contests this year) with current polling, you can see how sharply support for Clinton correlates. Vermont, Sanders's very-white home state, skews things a bit, but even when it is included, you can see clearly that the higher the percentage of the vote that is black, the more support Clinton gets.