Nowadays, we never allow ourselves the convenience of being temporarily unavailable, even to strangers. With telephone and beeper, people subject themselves to being instantly accessible to everyone at all times, and it is the person who refuses to be on call, rather than the importunate caller, who is considered rude.
Since the emergence of the Republican Party, only two Democratic presidents, Franklin Roosevelt and John Kennedy, have been followed by Democrats, and both FDR and JFK died in office, so their successors ran as incumbents.