Super Tuesday included a number of states in which black voters made up a large portion of the electorate, according to preliminary exit polls reported by CNN. Or, at least a large part of the Democratic electorate.
As in South Carolina, Clinton won the vast majority of votes from black voters.
In several states, as in South Carolina, Clinton beat the margins with black voters by which Barack Obama beat her in 2008.
That's Clinton's advantage in a nutshell. That's her advantage in the Democratic primary — even though Clinton also won the white vote in every state besides Oklahoma, Massachusetts and Vermont. It's also her advantage in a general election. Black voters vote heavily Democratic (as that first graph makes clear). The odds that any Republican will peel much of that group away from Clinton seems small — and in 2008 and 2012, black turnout in the general (as a percentage) topped that of whites.
Young voters could potentially have carried Bernie Sanders to victory. Instead, black voters carried Clinton.