From a NY Times article last March:
http://www.nytimes.com/2008/03/24/us/politics/24mccain.html?pagewanted=print In the spring of 2001, Mr. McCain was by most accounts still angry about the smear campaign that had been run against him when he was campaigning for the Republican presidential nomination in the South Carolina primary the previous year. He had long blamed the Bush campaign for spreading rumors in the state that he had fathered a black child out of wedlock, which Bush aides denied. Mr. McCain was also upset that the new White House had shut the door on hiring so many of his aides.
“Very few, if any, of John’s people made it into the administration,” Mr. Daschle later wrote in his book “Like No Other Time.” “John didn’t think that was right, that his staff should be penalized like that.”
Mr. McCain had begun to ally himself with the Democrats on a number of issues, and had told Mr. Daschle that he planned to vote against the Bush tax cuts, a centerpiece of the new president’s domestic agenda. Mr. McCain often made “disparaging comments” about Mr. Bush on the floor of the Senate, Mr. Daschle recalled.
Still, Democrats were stunned one Saturday in late March when, by their account, John Weaver, Mr. McCain’s longtime political strategist, reached out to Thomas J. Downey, a former Democratic congressman from Long Island who had become a lobbyist with powerful connections on Capitol Hill. In Mr. Downey’s telling, Mr. Weaver posed a question to him over lunch that left him stunned.
“He says, ‘John McCain is wondering why nobody’s ever approached him about switching parties, or becoming an independent and allying himself with the Democrats,’ ” Mr. Downey said in a recent interview. “My reaction was, ‘When I leave this lunch, your boss will be called by anybody you want him to be called by in the United States Senate.’ ”
I have a good friend who bleeds Republican and he refuses to vote for McCain because he thinks he is a Democrat. I think McCain is a chameleon who is will to change colors or say or do anything that will help him even if it contradicts an earlier statement or action of his. So whether this story is true or not, there are people who have always voted Republican who will not vote for McCain.