Bush fatigue was a huge factor in this election. But the nation was more than just tired of Bush - they were embarassed by him and angry at him for his many failures and the disasters he brought to the country by his incompetence and his disregard for the well being of the citizens.
From the War in Iraq, begun with a lie, to Katrina and his callous indifference to the lives of the citizens of New Orleans, to the financial meltdown that was largely due to conservative deregulation of the financial industry, Bush proved what a disaster one unqualified and incurious person can be in the White House.
No one, even his staunches supporters, believed McCain to be of high intellect - smarter than Bush perhaps, but still not someone with the kind of brilliant mind that the people, in their own common wisdom, knew was required to solve the many Bush problems that he will inherit.
Eight years is a long time to give to a president who did not earn the popular vote the first time around, and had to be appointed by the Supreme Court. Four more years is a long time to give the benefit of the doubt to someone whose first term was less than stellar.
Being of the same party, and voting with the president 90% of the time, did not help McCain win over the people. (In fact, it was only when he camapaigned with his choice for V.P., Sarah Palin, an extremist, albeit attractive know-nothing who appealed to the wing-nut religious base, that McCain even got big crowds.) Although McCain ran against Bush in 2000, and was the preferred choice of many because of his more independent status, once Bush smeared and defeated him in the primary, McCain became a loyalist and supported the president, banking on Bush being a successful president who would support him in 2008. Only by 2008 Bush was a miserable failure and only hurt McCain.
In the end, you might say Bush defeated McCain twice.
