So tell me if this works or is too much of a stretch:
Future Presidential candidate Bill Smith is a staunch isolationist. Anti-foreigners, especially anti-French, and is taking the lead in an on-going Franco-US dispute over the ownership of some islands in the Atlantic.
A historian he hired to put together his family tree as far back as possible has discovered that he has French blood. Royal French blood.
Question: if revealed would this be enough to derail his candidacy? Can you imagine blackmail? Murder? Or would people say, "Meh, so what?"