Perhaps the most important skill we learn in life is how we make decisions. Too little time is spent reflecting on this skill in ourselves, and evaluating it in leaders. How we see the world influences the decisions we make. If everything that happens is viewed through the lense of how it affects me personally, for example, does this help or hurt my chances for re-election, then most of the decisions I make are for my own self interest. (Note that very, very few people do NOT make any decisions in their self interest. I am talking about degree)
I suggest that over the course of history, those who history says are good leaders put common interest above their own in the moments that matter. For me the decision making process is a core competency of anyone who I consider to recognize as leader and who may gain my vote in an election.
Incumbents have the blessing or curse of having demonstrated how they make decisions, and thus tend to have an easier or sometimes bigger hurdle to overcome.