Trump’s delegate problems stem from two major issues. One is his lack of organization: Trump just recently hired a strategist to oversee his delegate-selection efforts; Cruz has been working on the process for months. The other is his lack of support from “party elites.” The people who attend state caucuses and conventions are mostly dyed-in-the-wool Republican regulars and insiders, a group that is vigorously opposed to Trump. Furthermore, some delegate slots are automatically given to party leaders and elected officials, another group that strongly opposes Trump, as evident in his lack of endorsements among them.
There are various ways these delegates could cause problems for Trump. The most obvious, as I mentioned, is if the convention goes to a second ballot because no candidate wins a majority on the first. Not all delegates become free instantaneously,2 but most do, and left to vote their personal preference, most of them will probably oppose Trump.
Conversely, Trump isn’t totally safe even if he locks up 1,237 delegates by the time the final Republicans vote. The delegates have a lot of power, both on the convention floor and in the various rules and credentials committees that will begin meeting before the convention officially begins. If they wanted to, the delegates could deploy a “nuclear option” on Trump and vote to unbind themselves on the first ballot, a strategy Ted Kennedy unsuccessfully pursued against Jimmy Carter in 1980.