Presidents are inevitably shaped by the circumstances in which they campaign for, and come into, office. In 1932 Franklin Roosevelt called for “bold, persistent experimentation” and followed through once in office. Had Roosevelt run in another year, or had there been no Great Depression, he would have campaigned and governed differently.

The same can be said, to varying degrees, of the presidents who won open-seat contests since, John Kennedy, Richard Nixon, the two George Bushes and Barack Obama, and of those who unseated incumbents, Jimmy Carter, Ronald Reagan and Bill Clinton.