My guess:

Clinton: hostile to poker just like Obama

Sanders: if he doesn't personally think it's wrong, he would probably support a nationalized site or maybe extremely regulated private ones where only the biggest two/three could exist.

Cruz: would not ban it federally, maybe would loosen federal attacks on it. Likely personally supports it.

Trump: would probably like it to be legal for only a handful of his biggest lobbyists, but would definitely ban it if Adelson paid him enough.