Who was left behind?

I am a West Wing fan. And in the first or second season, President Jed Bartlett was preparing to give the State of the Union address I learned something I didn't know, but realized made sense. There is one person in the government that is chosen to be left behind. Should something happen in the chamber where the address is being given (alright - I show my ignorance - I have no idea if that is the hall of congress or somewhere else) and no one makes it out alive, this person becomes the leader of the free world.

So I ask you, who was left behind last night? That is what I wonder the day after.