How does this happen? IOTL, Buchanan beat Fremont by 12% in the popular vote. Lets say that Franklin Pierce is renominated in a bitter convention fight, and Fremont manages to carry Illinois (4% Buchanan margin IOTL) and Pennsylvania (18% Buchanan margin IOTL, but Buchanan was from Pennsylvania, in addition to the states he carried, and gets an electoral college majority, even if he doesn't win the popular vote.
Somehow we get Fremont elected President in 1856. What happens after that?