On the possible effect of the enthusiasm gap on the representativeness of polls using registered vs. likely voter screens, I would suggest the following point for consideration.
If candidate A is generating a lot more enthusiasm among his supporters than Candidate B is among his own supporters, then it also seems likely that candidate A's supporters would be more likely to participate in polls. Rather than being underrepresented, then, Candidate A's supporters would perhaps be more likely to be overrepresented than Candidate B's. (I'm thinking of John Brehm's argument that participation in polls is akin to participating in politics, and so the same factors that predispose people to vote are likely to predispose them to consent to an interview.)
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