Under the law in effect when both Obama and Cruz were born, a child born abroad with one citizen parent and one noncitizen parent was a citizen at birth, provided that the citizen parent met certain residency requirements. One of those was having lived in the United States for at least five years after attaining the age of 14.
Obama's mother was only 18 when he was born, so she didn't meet that requirement. If she had, for some inexplicable reason, undertaken a lengthy and expensive trip, while pregnant, so as to give birth in the Third World instead of in a U.S. hospital where she lived, then her son would not have been a citizen at birth.
Cruz's mother, by contrast, was several years past her 19th birthday. She was born in the U.S., graduated from college here, and worked here for a while before she and her noncitizen husband moved to Canada. There is no serious question that she lived in the U.S. for at least five years after turning 14. Of course, it's possible that some Orly Taitz of the left might come forward and demand that she produce utility bills or rent checks from the 1960s to prove that point. The Constitution is silent about the procedure for determining eligibility.
The law has since been changed. Now, a child born abroad to a married woman is a citizen if either spouse is a citizen, without regard to residency. The answer was at one point more complicated if the woman was unmarried but I don't know the current provision on that score.
With regard to McCain, I think the whole business about a hospital on the base is a red herring. If some Panamanian woman had happened to go into labor near there and be rushed to that hospital, I don't think the child would have been considered a citizen. It simply wasn't a part of the United States for purposes of the Fourteenth Amendment.
The real answer is that McCain, unlike Obama and Cruz, did not have a noncitizen parent. Under current law, a child born abroad to two U.S. citizens is a citizen at birth. I think at one point the law was somewhat more restrictive and conferred citizenship in such instances only when one of the parents was in the military or diplomatic service of the United States -- I'm not sure about that. Your eligibility would depend on checking the law as it stood on the date of your birth. McCain's parents were both citizens and his father was in the military so McCain met even that tougher criterion.
I vaguely recall that there might actually be some semblance of a legitimate argument about McCain. (There is not even such a semblance in the cases of Obama or Cruz, or of Jindal, whom some of the extreme birthers also consider ineligible because his parents weren't citizens at the time of his birth in the United States.) The argument about McCain is that, as of the date of his birth, which you may recall was quite some time ago, the statute about citizenship hadn't yet been honed as well as it has been since, and there might have been some ambiguity in the provisions in effect back then. The law was amended after his birth and the amendment was stated to be retroactive, but there's a legitimate question about whether retroactivity can apply in this context. If McCain was not a citizen at birth, but was subsequently conferred citizenship, one could argue that he is now a citizen but not a natural-born citizen. I do know that one of the people who sued over Obama's eligibility also sued over McCain's, with both cases being thrown out of court.
Several years back there was noise about amending the Constitution to eliminate the requirement. The idea was that Democrats would support it to make Granholm eligible, while Republicans would feel the same way about a certain popular but Austrian-born Governor of California. (This was when Schwarzenegger was riding high in the polls.) Alas, nothing came of it.
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