McCain’s citizenship was never in doubt. As the child of two American citizens, he acquired citizenship at birth.
The question was whether or not he was a “natural born” citizen, as required by the Constitution for someone to be President. “Natural born” is more restrictive than simple citizenship.
There’s plenty of explanation of the difference between the two here on FR. Please read so you know the difference, because it is entirely possible that it will be a serious issue in the future.
McCain actually was questionable. He was not born in the Canal Zone, but was born over the line in a hospital in Panama.
You can argue that he deserves to be a full citizen since his father was in the Panama Canal Zone on active duty and his wife accompanied him. But for whatever reason she chose to give birth across the border in Panama.
The whole business is very suspicious. I think the Democrats raised the McCain issue largely in order to put future Obama issues to rest. The resolution passed in the Senate seems to be directed more at Obama than McCain in some of its details.