Don't get me wrong: Tenacity is certainly an important quality, and one to be respected. Without it, much that is truly worthwhile would never be achieved. It's a sad--and somewhat pathetic--mistake, however, to treat "not quitting" as an absolute. An understanding of the art of quitting, I believe, is as important as knowing how to win and when to declare victory. (An example of when not to declare victory: "Mission Accomplished"!) I checked in Sun Tzu's "The Art of War" to learn from this ancient and still unparalleled source of wisdom on the subject of human conflict, and found this quotation: "Winning one hundred victories in one hundred battles is not real excellence, winning a victory and subduing the enemy without fighting is the highest excellence." Sun Tzu adds, elsewhere: "Build your opponent a golden bridge to retreat across."
There's wisdom for both Hillary and Barack here. For Hillary, it suggests that winning victories is not enough in itself, and that fighting against all odds is not the only way to win. There's a bigger picture than the one she has recently been looking at, and in that picture the victory of the Democratic party in November is infinitely more important than a couple more senseless, ego-saving skirmishes between now and the Democratic convention. In this light, "quitting" could be seen as an act of selfless generosity, a victory for the many over the victory for one.
On the other hand, I trust that Obama is already working on that golden bridge for Hillary to retreat across. It's vital for her to be able to make that transition with dignity, and with the sense of having accomplished something important in her struggle. It's also vital that there be room, on that golden bridge, for those who worked and voted for her, with passion. In this case, the act of quitting might be seen not as surrender but as the better part of valor.