It's going to be great fun if she wins, because ex-US presidents continue to be called "Mr President" for the rest of their lives.
So it'll be "Madam President and Mr President" or even "Madam and Mr President"!
Off topic, but I wonder if the US of A will still be thought of as a democracy when Chelsea Clinton runs for President against Jenna Bush, Alexander Eisenhower, Rose Kennedy Schlossberg, Cara Kennedy Cuomo and Katherine Schwarzenegger