If Cruz or Trump win the necessary delegates in the primaries and caucuses, and the GOP denies then the nomination you can kiss the Republican Party officially goodbye from being a nationally viable political entity. If four million conservatives stayed home you can double maybe triple that number in the next election if this scenario occurs.
When it happens if Cruz and/or Trump don’t go immediately to a new party run that has had the groundwork already laid it doesn’t matter whether the republican Party crumbles or not. The whole country is then a rapidly declining and totalirarianizing place with no remedy. The killing will start in earnest shortly after.