On this day in 1856, President Franklin Pierce lost his bid for the Democratic Party's presidential nomination to James Buchanan, who would win the election later that year. Pierce, the 14th U.S. president, had served in the Mexican-American War. His support for the Kansas-Nebraska Act of 1854—a bill that allowed the organization of states out of the Nebraska Territory into states that could allow slavery, if they chose to—had become a flashpoint for opposition to his presidency.
Any states formed out of the unorganized Nebraska Territory weren't supposed to permit slavery as it was north of a line established by the 1820 Missouri Compromise, which established that only those new states south of the line could engage in slavery. The 1854 bill violated that agreement, instead allowing new states formed out of the Nebraska Territory to make that decision themselves.
The passage of the Kansas-Nebraska Act is now seen as a major driver of the division that would result in the U.S. Civil War. Pro- and anti-slavery advocates swarmed Kansas, hoping to establish the soon-to-be state according to their preferences on that issue. The violence between those groups became so notorious that the whole situation was called Bleeding Kansas.
That kind of mismanagement was one of several factors that sunk any effort by Pierce to be re-nominated as the Democratic Party candidate. Buchanan beat Pierce and the other contenders pretty easily.
Pierce handed over the keys to the White House in March 1857. He remains the only elected president who wanted to run for another term but was denied renomination by his own party. Four other presidents were similarly denied, but they had all been vice presidents finishing the term of an elected president that had died.
You can learn more about Pierce's troubled presidency and re-election effort at the University of Virginia's Miller Center.
