For the question of whether or not George Romney was a US citizen at birth, it doesn't matter where George Romney was born since he was born to two US citizen parents. For the question of whether he is a "natural-born" citizen, some argue that it does matter where George Romney was born. But whether George Romney was a "natural born citizen" as opposed to a "citizen" has no bearing on whether Mitt Romney is.
Mitt Romney was born in the US to two US citizens, so any suggestion that he's not a natural-born citizen is absurd on its face.
So do you think if George Romney had became the nominee the Senate would have had to get a resolution like they did for McCain stating he is a natural born citizen?
This was entered into the Congressional record in 1967:
Congressional Record (House) June 14, 1967
“I find no proper legal or historical basis on which to conclude that a person born outside of the United States could ever be eligible to occupy the Office of the President of the United States. In other words, In my opinion, Mr. George Romney of Michigan Is Ineligible to become President of the United States because he was born In Mexico and is, therefore, not a natural-born citizen as required by the United States Constitution.”