Rudy Giuliani's recent speech is a glimpse into the biggest reason why he is arguably going to win it all. He is a fighter, and he is willing to take it to opponents. Furthermore, he's proven he can handle things when the brown stuff hits the fan.
George W. Bush, for all he has gotten right, isn't a fighter by nature. If anything, he has turned the other cheek to a fault. Much like his father, he believes in trying to solve problems and in bringing people together. This does not make him a bad conservative, bad President, bad Republican, or bad person. But it has meant that a number of slanders and lies have gone unchallenged in his case, just as it led to his father's ill-advised compromise on taxes in 1990.