I don't think that Trump really tops out at 33% of likely republicans. He's benefited as much as Cruz from the drop in Carson's fortunes.
I think the racist and religious factions of the republican party comprise about 60%-66% of the total, currently divided between Trump, Carson and Cruz. I think that is the ceiling for Trump, provided the other two are pushed out the way. The business and warhawk factions, "the so-called" establishment, comprise the rest. It is pretty clear that they are in the minority. For years they provided the brains of the party, while the other two provided the heft. Now however, they are the victims of their own strategy. By encouraging republicans to be suspicious of 'elite experts' and telling them that compromise is wrong and that someone is 'either for you or against you,' they have set up the situation where they have lost control over the bulk of their party, which regards them as the very sort of elite, compromising RINOs that they themselves trained the masses to be suspicious of.
I'm becoming convinced that labels such as 'moderate' simply don't apply. Sure, Trump has a variety of views that don't square with certain 'conservative' voter preferences, yet those same 'conservative' voters back him. This seeming contradiction has baffled a large number of so-called pundits, who can't understand how their lazy, short hand categories have misled them. It shows that these terms are useless. When one ceases to think of such terms as 'moderate,' 'liberal,' and 'conservative,' and instead look at the various voting blocs that comprise the parties, everything becomes clearer.