When George W. Bush announced plans to invade Iraq in early 2002 millions of people were completely surprised. There was a strong attempt to get him to change his mind, including demonstrations by thousands of people across the world. By ignoring the voices of informed people and proceeding with a plan to invade he permanently alienated a large percentage of Americans. That he has many enemies was evident in the fierce campaign against Bush in 2004.
It is because of this that the respect given to John McCain is impressive. While the GOP was railed against by all number of progressive (and even conservative) grassroots movements John McCain seemed to be excluded from the unmetered hatred.
John McCain has criticized Bush on several occasions:
It is because of this shared criticism of Bush that John McCain could do very well against a Democratic nominee. Especially if that nominee is perceived as having betrayed liberals as Senator Clinton is believed to be doing.