Although the term "natural citizen" hasn't formally been defined, Ted Cruz was in fact born a citizen of the United States since his mother was an American citizen even though his birth took place in Canada. Some people may argue that to be a "natural" born American citizen, one's birth must have taken place here. I believe that is a question for the Supreme Court to decide.
The Constitution lists only three requirements a person must meet in order to be eligible for the job as President of the United States. The person must be over 35 years of age, have been a USA resident for at least 14 years, and have been a "natural born" citizen. Now, it is true that Cruz was born in Canada, but what does "natural born" really mean? I think that if McCain was able to run and be eligible even though he was born near Panama because his parents were stationed there for the military then Cruz should be considered eligible as well.
I don't agree with Cruz's politics nor his views, but technically he is eligible to become the next president of the United States. As per US citizenship laws, he inherited his mother's US citizenship even though he was born in Canada. Furthermore, I think the GOP would not jeopardize its status in the American political arena any more than it has done thus far, by supporting a candidate who could not rightfully win the presidency nomination.
Ted Cruz is not eligible to become President because of his divisiveness, not because of where he was born. His mother was a US citizen when she had him so he is a natural born American citizen, so that is a non-issue. He is a very calculating man though, who utilizes whatever is at his disposal to accomplish his goals and agenda. Remember, he was the one who single- handedly orchestrated shutting down the government and costing tax payers billions. He stood off to the side and let the other Republicans take the hit for it while he smiled to himself and planned his next strategy to get to the White House. He is by far the most dangerous politician I have ever seen, and I've been following politics since i was a child.