If Obama goes on to win South Carolina, there's a good chance California will be the next big showdown for the Democrats. It has far and away the most delegates up for grabs on February 5 (or any time for that matter). And you have to figure that the other Feb. 5 blockbuster states of New York and Illinois will basically be spoken for. Meanwhile, despite early polls showing Hillary up comfortably there, California's Democratic primary is open to independents, which has to be a source of comfort to Obama (and which are notoriously difficult to poll).
Which brings us to Hispanics, the big demographic variable in the California contest. If you look strictly at the demographic numbers, you find that roughly 35 percent of the state is Hispanic, versus only about 7 percent African American. But, of course, the more relevant question is what percentage of voters each group accounts for, which is somewhat closer.
According to state-level exit polls, Hispanics accounted for 19 percent of the vote in 2006, and 21 percent in 2004, versus 5 and 6 percent for African Americans, respectively. Keep in mind also that these are general election figures. The Hispanic numerical advantage is probably slightly lower in a Democratic primary, since Hispanics are far more likely to vote Republican than African Americans. (Still not super likely, but speaking relatively here...)
So, in all likelihood, Hispanics will make up somewhere between 10-15 percentage points more of the electorate than African Americans in California. That's a nice cushion if one candidate enjoys a big lead among Hispanics, as Hillary did in Nevada. But it's not quite the advantage you might expect looking only at demographics.
Update: Commenter virginiacentrist makes some plausible assumptions and works through the math to arrive at the following conclusion:
SC is 29% black and we estimate about 50% of dem primary voters will be black. That's a factor of 1.7.
NV is 24.4% hispanic, with a hispanic turnout of 15% in the primary. So that's' a factor of 0.6
If we extrapolate California:
CA is 7% black. So we can project 12% of dem primary turnout will be black.
CA is 36% hispanic. So we can project 22% of turnout will be hispanic.
If Obama wins 80% of the black vote, and Hillary wins 65% of the hispanic vote, then we'll have the following result (before the white vote is added on):
So...blacks and hispanics cancel each other out, and it comes down to the white vote in California!