Excellent question!
I'd guesstimate that one of the "thirds" have many crossover voters and the other of the "thirds" have many tea-party types; with exceptions both ways.
I hope it ends with the convention w/o 3rd party, and able to vote for Trump or Cruz.
We may be in a situation where 1/3 won’t vote TRUMP and another 1/3 won’t vote CRUZ, and another 1/3 won’t vote for a GOPe alternate from a brokered convention............