I was replying to a poster who did just that.
It is important to look only at polls that have two non-incumbents running. Those years were 1968, 1988 and 2000, I believe. People, deep down, are afraid of change, so unless the President is utterly incompetent, as Jimmy Carter was, or broke a promise to his base, as Bush senior did, they will, last minute, fall back on avoiding change.
In 1968, in a year of chaos, voters threw out the Democratic candidate, Hubert Humphrey, and opted for change.
In 2000, voters opted for change, despite peace and prosperity.