And I've been on record all along as liking Castro, so I'm not saying this as a criticism, though I do find it disappointingly predictable.

I think he's thinking about the VP spot. He's more interested in the executive branch (certainly doesn't want to come into conflict with his brother's career), he knows a Democrat won't win the Texas governorship anytime soon, but he wants a step up from being a cabinet secretary, and VP is the obvious step up.

He knows that whether we like it or not, the effort to hold our tenuous coalition together means that bare, simplistic demographics will make the call (as they really always have for most of our adult lives). If one of the male candidates wins the nomination, Castro knows that the runningmate will be a woman, so his shot ends there. Of the female candidates, we have (unless I'm forgetting somebody) Klobuchar, Williamson, and Warren. Klobuchar would have to name some young, millennial, "new progressive" dimwit to satiate the instagram activists. Williamson would name a space alien, which (according to some) might give Castro a shot. But I'm guessing Castro isn't actually a space alien; he's just kinda funny looking and has a cylon copy in the House. That leaves Warren.