The presumed theory of the case for Harris going after Biden on the busing issue is that, besides giving her an initial spike in the polling, it would pry black voters away from Biden, particularly in the South Carolina primary, the first primary where black voters will be important.
Well, there's no doubt that Harris has benefited in a general way from the busing hit, but in South Carolina perhaps less so. The recently-released Fox News poll of South Carolina voters has some very interesting data along these lines. (Note to those who would be inclined to dismiss a Fox News poll: this is a very good poll, coordinated by the excellent political scientist Daron Shaw and given an "A" rating by 538.)
In the poll, Biden is way ahead of the competition, with 35 percent first choice preferences to 14 percent for Sanders and just 12 percent for Harris. Even more interesting is the split among black voters, where Biden has 41 percent compared to 12 percent for Harris. Harris actually does better among white voters than black voters in this poll. Similarly, South Carolina black voters prefer Biden over Harris on racial issues, 28-18, while white voters are roughly the reverse.