The Constitution states:
“No title of nobility shall be granted by the United States: and no person holding any office of profit or trust under them, shall, without the consent of the Congress, accept of any present, emolument, office, or title, of any kind whatever, from any king, prince, or foreign state.” – Article 1, Section 9
Rudy Giuliani is running for the highest office in the land and he has been knighted by the Queen of England. This knighthood, called “Knight Commander of the British Empire”, was bestowed on Rudy Giuliani on February 13, 2002.
I had read about this clause before but was unaware that Giuliani actually had such a title. Like the author of the linked article writes – Giuliani must drop out of the race or repudiate his title from England.