Incumbent presidents generally try to cast their re-election contest as a choice between the imperfect but well-meaning and effective occupant of the White House and the far worse alternative offered by the rival party.
Challengers, on the other hand, try to frame a presidential race as a referendum on the sitting president whose record nearly always contains missteps, or who can be blamed for trouble in the economy or elsewhere.
In short, whether it's the president or the challenger, the way the game is played requires each to define the opposition as well as himself.