This notion comes from the pretty excellent 1991 Action Comics annual, an alternate timeline in which Superman finds himself running for President. (Clark Kent was Pete Ross' campaign manager who was critically injured in an assassination attempt in which Kent's identity was revealed.) To quote: "The justices [of the Superme Court] acknowledged S.T.A.R Labs' evidence that Superman had been artifically conceived within the birthing matrix Thus, it was their unanimous decision that the Man of Steel was truly born only after the matrix opened, on U.S. soil!" And Supes did turn out to be an awesome President.