There have been accusations of conspiracies involving the assassination of JFK and the assassination attempt on Ronald Reagan since they occurred. Of course these aren't true; Oswald acted alone, as did Hinckley. But we do have an example of an actual conspiracy to kill a president in the Lincoln assassination, in which Booth and his compatriots conspired to kill Lincoln, as well as other US government officials. What ensued was a massive man hunt and millions calling for blood, and accusations flying left and right.
So what would be the effect were there to have been actual conspiracies to assassinate President Kennedy or Reagan?