While discussing the incompetence of Trump in the New York Times, David Brooks mentioned the Dunning-Kruger effect. Also Politico writes about it. Wikipedia explains:

The Dunning–Kruger effect is a cognitive bias wherein persons of low ability suffer from illusory superiority, by mistakenly assessing their ability as greater than their actual capability.

More importantly in this context, and an important reason why we should select our leaders with a normal competence based vetting proces like a job application rather than elections, is that those who are compentent tend to underestimate their competence.

In a test performed with psychology students:

students of high ability tended to underestimate their relative competence. Roughly, participants who found tasks to be easy erroneously presumed that the tasks also must be easy for others; in other words, they assumed others were as competent as, if not more competent than, themselves.

And now think of what you a political candidate needs to do during a election campaign. Certainly not underestimate their competence! Luckily, political parties tend to preselect competent candidates and many political campaigns seem to be more about discussing the issues at hand rather than the competence of the candidate.