I do find it very unlikely that Biden is sitting at 35 unless Steyer's ads are stealing his voters. Nevertheless, it's something to consider if additional polls show him doing this poorly and Steyer doing this well in the state.
I don't think It's very unlikely if he's polling around 20 or 20+ in IA and NH and his national numbers are around 30. And we've had couple of polls before the holidays showing him closet to 25 nationaly.
National polls are useless when examining Southern black support, which has backed the inevitable winner of every Democratic presidential primary since 1992. The combination of share of the electorate and bloc-voting propensity more or less disconnects the modern South from national trends; the most recent example is how Sanders basically held Clinton to a stalemale outside the South, but lost due to losing the South 2:1 in delegates. Biden is and always was the favorite for this group (as I've been saying for 2+ years), with his floor in a field this large always being 45-50% (in a two-way race, it's more like 70% - and can easily go higher).
In SC specifically, given that black voters will likely comprise 55-60% of primary voters, it's unlikely Biden could be at 35% unless 1) Democratic primary racial polarization not seen in this century has somehow re-emerged or 2) Steyer's ads are having a substantial impact. It takes a
very special set of circumstances to break the dynamic of Southern black voters backing the candidate with the most establishment backing and/or name recognition, which tend to be one and the same (see 2008 for an example of what it takes to deconstruct this; even then, by the time SC was ready to fall from Clinton's grasp, Obama managed to achieve rapidly growing establishment support and comparable name rec, so it's not really even like he broke "the formula" so to speak).
