No person directly elected from the Senate has finished his term as President. So who we choose for Vice President becomes vitally important.
The first Senator to become President was Warren G Harding, the Bill Clinton of his day. Governor Calvin Coolidge of Mass. was his VP.
The second Senator to become President was Jack Kennedy, and of course LBJ was his VP.
No matter whether McCain, Clinton or Obama wins, they are all US Senators currently.
Charles the Curmudgeon.