At least for me because I got to know him.
When Ted Cruz blamed Donald Trump that started eroding my support of Ted Cruz.
TPP was a huge issue
I think from that point we had the Sea Island Meeting which Ted seem to accept that it , when we know it was the money men of the Democratic Party and consultants like Karl Rove, and Politicians. Little by little including I don't think he is a Natural Born Citizen.
He allowed the theft of Utah and Wisconsin. The people who Ted allowed him to used him with # Never Trump , he would have gotten a lot of support had he said the goal is #NeverHillary than a snake led by Mitt Romney or even told Mitt I want no part of Mitt his campaign would have been fine.
Colorado may have been the final straw. He didn't denounce it but laughed at stealing it.