I don't think we can say the possible voter pools are yet defined. Give it six months and this analysis will certainly be crucial, but until there have been a few more debates, many more events and endorsements and a lot more press coverage most of the pack is going to be little more than names (at best names and resumes) to a lot of the voters.

That's not to say that the polling will decisively change, but I think there's room for movement of second choices in particular, which may become crucial if some of the candidates drop out.