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.
Yeah, it would have perhaps happened. MI & FL would have halved her pledged deficit, but you also have to figure that in a world where they had been valid, the Obama campaign would have actually campaigned in MI/FL (and you know, actually bothered to put his name on the ballot in MI). In any case, that was the last hurrah; in the same sense that it was possible for the GOP to win with white voters alone in 2012. As you mentioned, the Dixiecrats are gone.
At the same time, it's easy to say "if she had only got a little bit more...", but the fact was that she
killed it with white voters in 2008. I just spent 30 minutes calculating it from exit polls out of curiosity...excluding caucuses,
she got 56% of the white vote nationally. Even in the 12 largest states (CA, TX, NY, FL, GA, NC, IL, VA, PA, MI, OH, NJ), she got 55% of the white vote. And it
still wasn't enough.
Even though the Dixiecrats were still somewhat of a presence, that display of "white solidarity" was unreasonably high. There was an obvious reason for that...had she been facing any other candidate, it wouldn't have been as strong and she might have had difficulties all the same (i.e.: more black votes, fewer white votes). Even if every single Dixiecrat from 2008 were still in the party, it would still be impossible barring a proxy race war that exceeded even the climate then for the white vote to be decisive.
White Democrats just don't have that kind of cultural or identity-based cohesion - in politics or in anything else, and especially not post-2008 - and any other group, while perhaps having more homogeneity in their voting patterns, just isn't a large enough bloc for it to really matter barring all of the other pieces of the puzzle falling into perfect place for an effective coin-toss.
And yeah: you might not need 40% of the black vote to win depending, which is why I said "nearly 40% [or more]". You
might be able to squeeze by with 35% if you are pulling close to Hillary-08 levels of white support
and doing well with non-black, non-white voters. My pre-SC formula for a national Sanders victory said "60% of non-blacks and 30% of blacks", which was unrealistic as all-get-out, but the pathway of getting 60% from non-blacks was still more likely than the pathway of him getting 35-40% among blacks.
Poll
