[Michael Michael]

Well, was McCain a citizen at birth, or wasn’t he...?

He became a US citizen by way of legislation passed after his birth. But it only made him a citizen by way if collective naturalization. To be a natural born citizen, you have to be born in the United States or its territories. And the Canal Zone was neither at the time of McCain's birth. Which is why 8 USC 1403 was passed, in 1937. To collectively naturalize those born in the Canal Zone to US citizens.

[Cyropaedia]

Constitutional scholars on the Right and Left agree that he was born a U.S. citizen. Both Ted Olson (former Solicitor General) and Harvard’s Lawrence Tribe have studied the case and agree at the very least that he is a citizen by birth.