Mainly, though, I think it would be a mistake to take candidates’ claims about this sort of thing too seriously. Suppose somebody said “policy outcomes over the long run are mostly determined by structural factors rather than election results; what’s more, it’s very unlikely that your vote — or your $250 contribution, or your time volunteering — can make a difference in the election.” Well, I think that’d be a much more accurate “theory of change” than anything I’ve heard from a presidential candidate, but ceteris paribus I’d still rather vote for someone with appealing rhetoric. That’s because a campaign’s not a seminar and the candidate’s job is to win. The candidates aren’t offering theories, they’re offering campaign messages. The theory is that a good message wins you the election and then you make your changes.
Scott Lemieux notes, for example, that “Bush in 2000, after all, didn’t campaign as a 50%+1 conservative who would increase party polarization in Congress, but that’s what he did.” I’ll just reiterate that on the big domestic policy issues, if you assume a Democratic win in 2008, the big determinant of what happens legislatively is the makeup of the Senate not the “theory” of the president. What’s more, insofar as “theory” matters, you can’t really infer anything from what people say in the middle of a primary campaign.