Via Brendan Nyhan, some evidence that Barack Obama has a ceiling of around 35 percent of the white vote in any given primary.

To me, the "ceiling" metaphor seems misleading in this instance. It's plain from the polling on Obama's favorable/unfavorable ratings that way more than 35 percent of white Democrats are well-disposed toward Obama. Since the knock on Hillary Clinton has tended to be that she's "polarizing" people forget that that's a two-way street -- lots and lots of people really really like Hillary Clinton which makes her hard to beat in a primary.