It is easy to define the Democratic Primary into three or four key questions. The Republican Primary is an almost infinite number of questions. However, they ultimately come down into several questions repeated over and over again — who makes it to Mid-March and when do other candidates drop out.
Right now Trump has a solid lead in the majority of national polls. While every state has some discretion over their rules, for the states within the two-week mandatory proportionality window, only Trump is safely over the 20% that states are allowed to set as a threshold for delegates. Additionally, when you add the other “non-politician” candidates, about 50% of the primary votes appears to be going to “outsider” candidates.
More significantly, there is little or no meaningful gap between a large block of candidates. There are currently five candidates with between 5-10% of the vote in the Real Clear Politics average of polls. Right now, it is easier to define who will almost certainly not make it to March 1 (Graham, Jindal, Pataki, and Gilmore) then to guess who will emerge from the pack to save the party from Donald Trump.
Barring some candidate committing a major blunder (and, what the media considers a major blunder appears to be very different from what the Republican primary voter considers to be a blunder as shown by Trump getting even stronger over the last month), it looks like there might be ten or more candidates that get to Iowa. And the more candidates that get to Iowa, the less that it will take out of Iowa to justify continuing (particularly for candidates like Bush and Rubio that do not mesh well with the base of the Iowa Republican party but are likely to do better in later states).
In short, heading into the Labor Day weekend, while the Democratic race seem relatively straightforward barring some major “game changing event.” The Republican race desperately needs some significant event to simplify the race. If not they could find themselves in late March facing a three or four candidate race with Donald Trump having a large delegate lead.