There were 68 purported “game changers” in the 2012 general election, most of which turned out to be irrelevant. But for the political observer trying to sift faux game changers from genuine twists in the campaign, the primaries present a couple of additional complications.
First, there are far more opportunities to be “surprised” in the primaries. Let’s start with the most basic stuff. In a nomination race, there might be a dozen or more candidates, instead of just two. And states vote one at a time, instead of all at once.
Furthermore, in a nomination race, there is an abundance of metrics by which you might judge the campaigns: national polls, Iowa polls, New Hampshire polls, favorability ratings, endorsements, fundraising, staffing, even crowd sizes and yard signs. Eventually, we’ll also be able to look at delegates, which can be counted in many different ways. For any of these metrics, you can report on the level of support (“Hillary Clinton is polling at 48 percent”), the trend (“she’s lost 4 percentage points since last month”), or even the second derivative (“she’s losing ground, but not as quickly as before”). Multiply 23 candidates3 by 10 metrics by three ways of reporting on those metrics, and you have 690 opportunities to be “surprised” at any given time.
...Here is another difference: a presidential race costs hundreds of millions of dollars, and very, very few people can pay for one out of pocket. Trump is worth about $4 billion. Although he is "merely" the 405th richest person in the world,he could meet the cover charge. It is highly unusual for such a person to seek a presidential nomination. Mitt Romney and Steve Forbes are both wealthy, but neither is in the billionaire range -- and neither could have afforded to pay his own way through the November election. Perot is a billionaire but he ran as an independent. One has to go back to Nelson Rockefeller in 1968 to find a nomination candidate in Trump's league -- and Rocky ran in the pre-reform era.
None of this is to imply that nominations are all that easy to forecast. And some things this year have been genuinely surprising. In particular, that there are 17 Republican candidates, including a dozen or so who have traditional credentials for the White House, is unprecedented. If you want to develop a theory about how “this time is different,” figuring out how to explain the size of the Republican field (and how it might affect the race) seems like a good starting point.
Why the paucity of billionaire contenders? For starters, there aren't that many billionaires: only about 536 in the United States, most of whom have no political ambitions. They play more as funders than candidates.