(CNN) As they campaigned over New York in recent days, Bernie Sanders and Hillary Clinton found themselves in an escalating war of words over which of them was "qualified" to be President. The dust-up has died down, and the candidates have moved on--but the issue is now on the table. What does make a candidate qualified to be President? CNN Opinion asked contributors who have worked in or studied presidential administrations, some with a front row seat, for their take. The opinions expressed in this commentary are solely those of the authors. David Gergen: The ability to collaborate Of the many qualities required for presidential leadership, three seem especially important in coming years: Character: Historian David McCullough argues that character is the single most important quality of a president. That seems true at a time when Americans are so divided and distrustful. Executive capacity: A president must not only have principle...