The party building peaked in 2002 when he even had blacks and Hispanics supporting him over Sept 11. After that is was damage control. Iraq was his undoing.
He did pull a win in 2004, based on many optimistic Iraq projections, and buying off a couple of key voting groups with borrowed money. But that was only a short term strategy, from 2005-2008 it hit the fan with everything going wrong, one after another with the worst at the end, TARP crisis. There was nothing to build on, he had a 20% base left, if that. He picked a path of no return.
Yes, at the end of eight years of Bush, I doubt that there was single mainstream Republican who could have beaten Obama with any assurance -- even many traditional Republican voters abandoned the Party.
It's as though the Parties decide to swap power every eight years, then just run show candidates to assure that predetermined outcome. This way they aren't competing, just cooperating, so each guy in office gets to further it's own big-government, globalist agenda. It's a lot like like the elections used to be in communist countries.