What U.S. President said “It's fiction, stupid!”?

What U.S. President said “It's fiction, stupid!”?

In the epilogue to his final book in the “Space Odyssey” saga: 3001: The Final Odyssey (1997), Arthur C. Clarke writes:

And if any readers of the earlier books feel disoriented by such transmutations, I hope I can dissuade them from sending me angry letters of denunciation by adopting one of the more endearing remarks of a certain U.S. President: "It's fiction, stupid!"

What U.S. President is he referring to and what is the context of the original quote? I'm unable to find anything other than circular references back to this epilogue by Clarke.

[Posted here deliberately and not in https://scifi.stackexchange.com/ because the quote is referencing a real President, not a fictional one (as far as I can tell) - in other words, other than that Clarke wrote it and in the epilogue to a scifi book, it doesn't seem to relate to scifi, but rather a presidential quote]