We all know that two presidents, Abraham Lincoln and James Garfield, were assassinated in the 19th century. But other presidents survived attempts to kill them, and conspiracy theories at the time, and surviving to the present day, surround some of those incidents.
There's no doubt that Andrew Jackson survived an assassination attempt, as the enraged president physically attacked the man who had just tried to shoot him.
Two other cases, which relate to tensions in the period just prior to the Civil War, are less clear. But people believed at the time that assassins had tried to kill James Buchanan in 1857. And it's conceivable that an attempt to kill Abraham Lincoln before he could take office was thwarted by some clever detective work.