One of those variables is the left-right ideology of the candidate, which I do include in my model and which political scientists have sometimes included in the past...It does not seem plausible, meanwhile, as some political scientists’ models imply, that the difference between Representative Michele Bachmann or Mitt Romney would amount to only 1 or 2 points at the polls.Well, of course it does not seem plausible. Romney is a normal presidential candidate with reasonably normal credentials who has shown fairly strong campaign skills and is probably on his way to winning a nomination fairly easily despite in some ways being a fairly bad fit for his party in ways that shouldn't much harm him in the general election. Michele Bachmann is a fringe crackpot who talks about a worldwide nuclear war against Israel and the socialist plot to replace Medicare with market-based insurance and how if we'd all emulate Communist China we'd be far better off; she's such a fringe figure that despite being a current Member of the House she hasn't managed to score a single endorsement from her colleagues there. Yes, a bit of that might properly be classified as ideology, but not most of it.
We can analyze the effects of ideology on presidential vote because we have examples to work from. There is no example of a major party in modern times coming even vaguely close to nominating someone without conventional credentials such as Bachmann or Prince Herman, or even a thrice-married, scandal-tarred former Speaker who has been out of office for over a decade and has ever-multiplying skeletons in his various closets. So we really don't know what kind of effect that would have on the general election, and it's very plausible to believe it could be far higher than the normal apparent penalty for ideological extremism. Or not! After all, we would be in a world in which someone, for example, who apparently knows virtually nothing about US foreign policy and national security would be able to win a major party nomination, and who knows what they world would be like?