It has happened four times (Franklin Pierce, Millard Fillmore, Andrew Johnson, and Chester A. Arthur), but three of them weren't elected president in the first

It has happened four times (Franklin Pierce, Millard Fillmore, Andrew Johnson, and Chester A. Arthur), but three of them weren't elected president in the first place (got there via deaths) and all of them happened before party primaries were decided by voters. http://wiki.answers.com/Q/Has_an_incumbent_president_ever_lost_the_nomination_to_run_for_a_second_term http://wiki.answers.com/Q/Has_an_incumbent_president_ever_lo...