David Gergen, an advisor to four U.S. presidents and currently director of the Center for Public Leadership at Harvard’s John F. Kennedy School of Government, doesn’t care as much about who wins the U.S. presidential election in November as much as he does about whether “the winner can be an effective leader.” The nominees, Republican Sen. John McCain and Democrat Sen. Barack Obama, each has strengths that he finds encouraging, and both have weaknesses that worry him, Gergen told his audience at the 12 th annual Wharton Leadership Conference , sponsored by the Center for Leadership and Change Management, and the Center for Human Resources. For this election in particular, Gergen said, the winner must handle some of the roughest terrain — foreign and domestic — since Franklin D. Roosevelt in the early 1930s. “In my judgment, the challenges facing the next president will be the most daunting facing any president of our lifetime.…