The guy who prevented the election of John McCain

Toward the end of Quantum Leap, whazisname the leaping dude, the later captain of the Enterprise, leaps into Lee Oswald, but can't stop the assassination of JFK. Managed to change history, though, in a way that was kind-of a nice touch for sf.

In the same vein, thanks to that guy who went back and prevented the election of John McCain, we don't know how bad it really could have been.