You have said that outside of the fundamentals the biggest variable in an election is the quality of the candidate. What qualities make a candidate a good one?And Colby asks:
[D]o we have any evidence that "authenticity" is really important electorally?The big thing here is that it depends a lot on which types of elections we're talking about. In general elections for president, the candidates almost certainly don't matter very much at all. There's some evidence that ideological extremism (Goldwater, McGovern) is punished, although even then not by large amounts. Beyond that, anything is going to be very small. Of course, at least in modern times it's hard to measure this because no one makes it that far without having won a nomination fight in which large electorates are involved...we don't really know how a truly inept presidential nominee would do.
In presidential nomination contests -- in all primary elections -- candidates of course are much more important. There are other things that can matter...some of it depends on how you conceptualize "candidate" as a variable. That is, people may vote by faction, ethnicity, or geographic region; a candidate may be a cue to those things.
Congressional (general) elections are where the academic literature talks about candidate quality a lot. Obviously, party matters there quite a bit. But because there sometimes is a very large gap in candidate quality (unlike, say, presidential general elections), it turns out that it matters: challengers with previous experience in elective office do much better than do those without such experience. Quality challengers will (all else equal) raise much more money and run professional campaigns, thereby closing the gap in name recognition and other factors that drive vote. What's less clear (and I'm not fully up to date on the literature, so maybe someone will jump in) is exactly why experienced candidates do better -- it could be that they have learned how to campaign, or it could be that it's easier to amass electioneering resources if more people think that you'll be a strong candidate (even if you actually are no better than some first-timer).
To turn to the question of particular traits such as "authenticity" -- I think you can see the circumstances where anything about the candidate can matter. Out of that, image is only one of many possibly relevant candidate traits. Out of image, authenticity is only one aspect. So it's not impossible at all that such things can matter...but almost every time you hear that it did, it's just a post-facto rationalization, as are most -- but again, not all -- discussions of campaign effects. If it's going to matter, though, I'd say that high-information primary elections would be the place to look.