George Romney was born in Mexico but he was never a Mexican citizen, any more than Juan McCain was ever a citizen of Panama. George Romney even ran for President of the United States—I don’t recall anyone saying he wasn’t eligible.
George was a Mexican citizen till the day he died. If he was ever nationalized as a US citizen, the evidence has not been made public. Us citizenship wasn’t automatic to children of US parents born on foreign soil at that time.
Mittens is eligible for dual citizenship by having a Mexican father. All he has to do is fill out a form.