As a baseline, voters should want a president to be rock-solid as a trusted source of command and control. While the incumbent may deviate to accommodate the circumstances, the center point is being dependable. By contrast, being erratic is not what is wanted.
In the instance of Donald Trump, Americans have witnessed personality extremes that might constitute erratic behavior. Not being a social scientist or psychologist, this analyst can only observe what is apparent. That is why in my new book, How to Select an American President by James George with James Rodger (c) 2016 Archway Publishing, I recommend that presidential candidates be evaluated by medical and mental health professionals to obtain a certified assessment. Having been a Fortune 500 Company manager in the past, I have personally had such psychological assessments, and they served as a guide to my employers as well as for my personal and professional development.
For a long time, critics have pressed Hillary Clinton about her changeability and inconsistencies with some alleged that she can't be trusted. Stripping away the political spin, overall, what one might see is adaptability.
Adaptability is a feature that may be applied to systems and people. In business as in government, being adaptable can be a good thing, as being flexible and accommodating. These characteristics are aligned with capacity and capability to collaborate and to compromise in a bipartisan environment that is our federal government.
Being rigid and hard-nosed are more autocratic behaviors that are less suited for working in an adaptive pluralistic democratic republic.
In addition to having acquired vast knowledge about a broad range of subjects, including to a working level of detail, an effective president needs exceptional personality traits.