The casual observer might have missed the subtext to Sunday night's Democratic debate, though it would take some missing. Hosted by the Congressional Black Caucus Institute in South Carolina on the day before the Martin Luther King Jr. holiday, the debate devoted a decent amount of energy to issues of race, including disparities in criminal sentencing.

In the lead-up to the event, we noted that there's a deep split in the Democratic Party along racial lines. Hillary Clinton and Bernie Sanders run about evenly with white voters, but among nonwhite voters, Clinton has a big lead -- one that hasn't diminished much even as Sanders becomes better known.

We've noted repeatedly that attitudes can shift quickly once voting actually begins. In 2008, support for Clinton among black voters eroded swiftly as it became obvious that Barack Obama could win the nomination. But, of course, Obama had a connection to the black community that Sanders lacks.

If the gap above doesn't close, it's problematic for Sanders over the long term. Many of the states with the most delegates are also ones with large nonwhite populations.

(Most of the bars below show the percent of the Democratic primary vote in 2008 that was nonwhite. For the states for which that data isn’t available, the percentage of the entire state that is not white is shown. Since nonwhites lean Democratic, the percentage of Democratic voters that is nonwhite would likely be higher than these census-based percentages.)

As we noted on Sunday, that becomes a problem for Sanders very quickly. The next two contests after Iowa and New Hampshire are in states with much larger nonwhite populations. And on Super Tuesday (March 1), that's even more the case.

Things can and will change quickly, and Sanders doesn't need to win the nonwhite vote to win the nomination. If Sanders and Clinton remain closely matched among white voters, though, Clinton's big advantage with everyone else could doom the Vermont senator's efforts.