This is the real answer, but if we want to be less cheeky about it, then:
February 27 / South Carolina primary. This was the night that Sanders demonstrated that he was never going to be able to command enough support within the black community to win the nomination.
The black vote now controls the Democratic Party. This has been demonstrated in the past two Democratic primaries and the trend-lines associated with all socioeconomic groups in the party do not show this dynamic becoming any less so in the near future. Whoever any more than a nominal majority of black voters supports will become the nominee.
There is no other voting bloc - racial or otherwise - that matters at this point, because there is no other subset of the Democratic voting bloc that has anywhere near the combination of size and homogeneity that the black vote does. If a candidate can't get nearly 40% or more of the black vote, then they ain't winning the nomination.
If Hillary did better in white caucus states in 08 and MI/FL had their full delegations seated, she likely would've won, right?
But yeah, that possible ship has long sailed with the departure of the Dixiecrats and the increasing dominance of the black vote in Democratic primaries. I don't think you'd necessarily need as high as 40%, but you can't be consistently getting blown out by 50-70 point margins either. It will be very interesting to see how the black vote goes in the next primary when there's not an Obama or a Clinton on the ballot. I'd imagine it would still be relatively homogenous, but not nearly as much so.
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