My answer to What qualities would a truly great US President have?
Answer by Ken Larson:
Character is by far the most important.
It is the only measure we have in the current political climate that the individual will shed his or her campaign and rise to the occasion of managing under very challenging circumstances.
“Presidents make history, but not on their own terms. They are constrained and harried on all sides by reality.
In selecting a president, it is important to remember that candidates will say what they need to say to be elected, but even when they say what they mean, they will not necessarily be able to pursue their goals. The choice to do so simply isn't up to them.
The American presidency is designed to disappoint. Each candidate must promise things that are beyond his power to deliver. No candidate could expect to be elected by emphasizing how little power the office actually has and how voters should therefore expect little from him. So candidates promise great, transformative programs. What the winner actually can deliver depends upon what other institutions, nations and reality will allow him. Though the gap between promises and realities destroys immodest candidates, from the founding fathers' point of view, it protects the republic. They distrusted government in general and the office of the president in particular.
Congress, the Supreme Court and the Federal Reserve Board all circumscribe the president's power over domestic life. This and the authority of the states greatly limit the president's power, just as the country's founders intended. To achieve anything substantial, the president must create a coalition of political interests to shape decision-making in other branches of the government. Yet at the same time — and this is the main paradox of American political culture — the presidency is seen as a decisive institution and the person holding that office is seen as being of overriding importance.
In the end, though, the U.S. presidency was designed to limit the president's ability to rule. He can at most guide, and frequently he cannot even do that. Putting the presidency in perspective allows us to keep our debates in perspective as well.”