Gregory Rodriguez in the Los Angeles Times today takes aim at the emerging conventional wisdom that Latinos won't support an African-American candidate, pointing to examples of Latino-backed black politicians from Harold Washington to Wellington Webb to Charlie Rangel. Beyond this, it's worth keeping in mind that even if Rodriguez is wrong, it's hard to see Latinos providing Hillary a "firewall" in California. Though Latinos account for 35 percent of the state's population, they (unfortunately) vote in very low numbers, in part because many aren't citizens.
In the 2004 Democratic primary, according to the CNN exit poll, Latinos accounted for 16 percent of the electorate, African-Americans 8 percent. Since then Latinos have grown as a share of the population, but presumably black turnout will be substantially higher this time. For the sake of argument, suppose the figures this year are 18 percent and 9 percent, respectively. Suppose further that Hillary wins the Latino vote 60-40, and Obama wins the black vote 75-25. By my quick calculation, this would leave Obama with 13.95 percent of the total primary vote and Hillary with 13.05 percent; the outcome of the race would essentially come down to the white (and Asian, Native American, etc.) vote. Obviously these are just rough figures, but the point is this is nowhere close to what anyone could call a "firewall".
Hillary remains a slight favorite to win the statewide popular vote, but there are a few confounding factors to consider. One is that, because the state GOP is dominated by hardcore conservatives who distrust independents, independents are allowed to vote in the Democratic primary but not in the Republican primary. Presumably this will provide a boost to Obama that the polls aren't picking up, but no one really knows what the magnitude of the effect will be. Second, keep in mind that a majority of the state's delegates are apportioned by congressional district. This would seem to help Hillary in that there are a lot of Latino-heavy districts where she'll pick up delegates no matter how low turnout is. Similarly, if Obama can replicate his Nevada performance in dominating the more conservative areas of the state, he can pick up delegates in California's twenty Republican congressional districts.
Update: It seems the Prince of Darkness made many of these same points in his column today.