‘as far as I know, his father was an American citizen when he was born.’
But Romney’s father, George, was born in Mexico. Some say that automatically made his father a Mexican and he has to naturalize to be a US citizen upon his return to US.
Some say since his father, George, was born to US citizen parents, he is a US citizen no matter where he was born - even Mexico.
So it is not quite clear if George was a US citizen when Mitt was born.
I think the argument that George Romney wasn’t an American citizen is without merit, and will fizzle upon close inspection.