The reasons for his Black support are incumbency, which always provides an advantage, media visibility, which affects uneducated voters (that's most of the voting public,) and his connections with Booker, a visible, Black politician with a pseudo-celebrity status. None of these will be easily translatable on the national stage, since he won't be an incumbent (and if he's running against Hillary, she'll actually be able to net some of that incumbent aura,) Booker won't endorse him over whoever the Democratic candidate is/the Dem candidate will have the support of other hyper-visible Black leaders, Obama included, and his media status won't be significantly greater than the Dem candidate, especially if it's Hillary.