Cornelius Fudge is another bumbling, minor character (like Mrs. Figg) who takes on new characteristics in Book 5. He is Minister for Magic and thus, head of the wizarding government in Britain. He has always seemed vaguely well-intentioned but ineffectual: he greets Harry at the Three Broomsticks when Harry turns up in Diagon Alley after running away from the Dursleys in Book 3, and he gets involved in some amusing cross-cultural misunderstandings with the Bulgarian Minister for Magic in Book 4.
Then, when Harry joins the Order of the Phoenix at Number Twelve, Grimmauld Place, he hears that Fudge has taken a turn to the dark side. After years of relying on Professor Dumbledore's assistance in making decisions, Fudge has grown to love power. He wants to hang on to the office of Minister for Magic at any cost. He begins to see Dumbledore as a threat to his authority. What is more, Fudge is entirely in denial about the return of Voldemort. He refuses to admit that this threat to the security of wizarding Britain has returned. So Fudge turns against Harry, as well.
In order to consolidate his own power, Fudge encourages the Daily Prophet to begin an ugly smear campaign against Harry and Professor Dumbledore so no one takes them seriously any more. He also holds a hearing regarding Harry's use of the Patronus charm in Little Whinging, a Muggle part of Surrey. He appoints vicious Professor Umbridge to Hogwarts to monitor Professor Dumbledore's actions. And when Professor Umbridge uncovers Harry's actions with the D.A., Fudge is on hand to attempt to expel Harry from school. When Professor Dumbledore takes the fall for Harry, Fudge tries to have his old mentor arrested.
Fudge's turn from vaguely amusing inefficiency to outright self-serving corruption is definite proof of the old saying that all power corrupts. As Remus comments:
Deep down, Fudge knows Dumbledore's much cleverer than he is, a much more powerful wizard, and in the early days of his Ministry he was forever asking Dumbledore for help and advice [...] But it seems he's become fond of power, and much more confident. He loves being Minister for Magic and he's managed to convince himself that he's the clever one and Dumledore's simply stirring up trouble for the sake of it. (5.184)
Fudge is a terrific example of what happens when a foolish, ineffectual person is given too much power: he is not on Voldemort's side at all, but he is so competitive and jealous of Dumbledore that he does almost as much damage to the good side as Voldemort does in Book 5. Fudge's denial gives Voldemort a chance to build up power without fear of Ministry interference for almost a full year – so Fudge really has a lot to answer for.
At the end of the novel, Fudge sees Voldemort attacking Professor Dumbledore in the Ministry of Magic with his own two eyes. His strict denial shatters, and he admits to the Daily Prophet that, "Lord – well, you know who I mean – is alive and among us again [...] We believe the demenotrs are currently taking direction from Lord – Thingy" (38.2). Having been forced to acknowledge that Professor Dumbledore is right and Voldemort is back, Fudge seems like a broken man by the end of Order of the Phoenix. In the final chapter, Fudge's inability to call Voldemort by his name – something that both Harry and Professor Dumbledore have the strength of character to do – underlines his continued weakness of character and cowardly nature.