Howard Megdal speculates:
[A] combination of the lack of strength within the current field, the inclinations of current GOP primary voters, the recently changed 2012 GOP rules, and even the best interests of the party itself all point to a strong possibility that there will be no presumptive nominee ahead of the convention.
[Megdal's argument is] based, to a large degree, on a complete misunderstanding of the latest reform to the nomination process, which claims that Republican delegates in early states will be apportioned by proportional representation. But that reform, or at least that interpretation of it, is simply a myth, as Josh Putnam has explained. Only some delegates in early states will be chosen using p.r. A lot of them will still be winner-take-all — not by state, but by congressional district. Meglal uses Missouri as an example, but in fact only half of Missouri's delegates will be chosen by p.r.