Giuliani, I believe, was leading 4 years ago. Giuliani fell because of personal issues that make him unelectable. You can go back to threads of that day to see what is meant, but the short of it is serial women, drag parades, and general social behavior and policies that rendered him dead in the water as soon as anyone really thought about it.
He was leading because of the sympathetic number of repubs who were thinking of 9/11 rather than of electability.
He remains, however, the single best public speaker in all of US politics. It is a shame he is not a conservative.
Good synopsis of Rudy and his falling out of the race. One disagreement with your post: Rudy was never a solid leader in the national polls, as for example Perry has been for a month, and Romney before that. He, or others, could take the lead in some states, but McCain held the balance (more’s the pity).