Did he not possess enough understanding of the qualities of both leadership and human nature to understand that by having Trump in the audience, the greatest putdown he could have achieved with such a man would have been to fail to mention him at all?
By his petty digs, he exposed his greatest vulnerability--naivete that is beyond belief in not understanding the nature of opponents. In this case, Trump. Further, he reinforced the already-prevalent public opinion that he is thin-skinned and hypersensitive, neither of which is a quality of a great leader.
If he underestimates such an opponent in America, how can he be trusted to make wise judgments in analyzing and dealing with foreign leaders?
He provided the weapon. Conservatives should attack him on that, attacking his inability to handle criticism and challenge, and lack of understanding of human nature.
Can anyone imagine Washington, Jefferson, Lincoln, or even Reagan handling such a situation as he did? The most Reagan might have done would have been to make one wry comment, and move on to self-deprecating humor (which is the usual pattern for the event anyway).
Herman Cain or Allen West are the only folks who appear to have what it takes to make the kind of bold and unrelenting personal attack which is needed to expose him, while, at the same time, being able to rebut the certain attacks most others would face of "racism." And, both of these gentlemen understand leadership and accountability, accompanied by a deep respect for and love of the principles of the Constitution they would be pledging to uphold.
But it is a trait of dictators like Hitler, Stalin, Castro and Chavez. Just to name a few.