The Democratic party is mostly about identity politics. We just can’t see the base accepting an entirely non-white national ticket; of course having Hillary Clinton at the top checks off some of the identity boxes, but this probably isn’t enough.
We predict that Clinton’s running mate will be one of the four following individuals: Secretary of Housing and Urban Development Julian Castro, whom the party has been grooming for a national role; New Jersey Senator Cory Booker; former Massachusetts Governor Deval Patrick; or Secretary of Labor Thomas Perez.
The latter brings the advantage of credibility as a hard-left activist. The two blacks on the list have portrayed themselves as more moderate, “clean” (as Joe Biden might say), modern black politicians; that is to say, geared toward acceptance by whites.
A Hispanic (even one who doesn’t speak Spanish) would also bring the advantage of novelty. The party has done the black thing, and now the woman thing.