His grandfather or great-grandfather defected, His father was born in Mexico, with no claim to U.S. citizenship.
Your premise is false, despite your apparent sincerity of belief.
George Romney was a US citizen at birth, (not a NBC however because he was born in Mexico) by virtue of the fact that BOTH his parents (Gaskell Romney and Anna Amelia Pratt) were US citizens (both NBC) and neither of them ever lost or disavowed their US citizenship.