"Umbridge first appears in Harry Potter and the Order of the Phoenix, as Senior Undersecretary to the then-Minister for Magic, Cornelius Fudge. As a senior undersecretary, she is one of Harry Potter's interrogators in Courtroom Ten as part of the Wizengamot, the wizarding judicial committee, when he is tried on charges of breaking the ban on underage sorcery. It is later revealed that Umbridge herself had ordered Dementors to attack Harry in an attempt to silence or frame him. He repelled them using the Patronus Charm and was thus entrapped into using magic for self-defence, which would have worked to Umbridge's advantage had it not been for Arabella Figg's testimony, Albus Dumbledore's defence and Amelia Bones's fairness." (http://en.wikipedia.org/wiki/Dolores_Umbridge)

"Fudge is particularly paranoid that Dumbledore is a threat to his power, and that he is, in effect, planning to train the Hogwarts students loyalty to himself rather than the Ministry and in effect overthrow Fudge. Ultimately Fudge manages to have Dumbledore removed as Headmaster and Umbridge put in his place, but this is overturned after the Battle of the Ministry, which proves Voldemort's return to power and destroys Fudge's political career.

When his failure to admit Voldemort's return comes to light, Fudge is forced to resign as Minister for Magic and replaced by Rufus Scrimgeour in Harry Potter and the Half-Blood Prince, though he stays on as a powerless advisor.

According to Hagrid, after Millicent Bagnold (Fudge's predecessor) left office, Dumbledore was the most popular man to succeed her, but refused himself, preferring to remain at Hogwarts. Fudge thus knew for his entire tenure that he was less popular, and also apparently that he was less qualified---Dumbledore was apparently an informal advisor to Fudge, with Hagrid claiming Fudge sent numerous owls asking Dumbledore for advice. This knowledge of Dumbledore's superior popularity and skill was likely one of the reasons for his actions after Voldemort's rise; in addition, it is implied that Fudge simply did not want to believe that Voldemort could return, knowing that that would be the end of the Wizarding world's years of peace, and decided to merely ignore all of the evidence rather than accept the truth. Fudge could thus be based loosely on Neville Chamberlain, the British Prime Minister who attempted to ignore Adolf Hitler's rise to power, believing that doing so would prevent war, while Scrimgeour could be seen as Winston Churchill." (http://en.wikipedia.org/wiki/Cornelius_Fudge)