Senator McCain is a Republican. But his biggest strength has always been his integrity and his honesty in voicing his views wherever they fall in the political spectrum.
I'm a Democrat. Until recently, I had been thinking that I could see voting for him as President specifically for the above reasons. I believed that his integrity would lead him to make the right decisions regardless of its level of support by his own party. I saw in him the potential to heal the Red State/Blue State divide that has created so many problems. He stood to get my vote because of his integrity not his party affiliation.
There is a great scene in the movie "Flags of Our Fathers" where John McCain's father visits West Point to check up on how his son is doing. He finds out that he is not doing so well academically but seems disinterested. He then inquires whether the younger McCain has done anything dishonorable... anything that would be considered "conduct unbecoming of an officer." Although this is of less interest to the academic, he says that McCain has conducted himself with integrity. His father makes it clear that this is all that matters and leaves. It is this integrity of character that clearly defines the rest of the movie and McCain's behavior in real life.
But in the last few months, he has lost a bit of this integrity. He has been slowly but steadily shifting opinion on a number of core political issues. All of it appears to be aimed at shoring up support from Bush's core constincuency... the religious right and political conservatives. But it is exactly this move that is making him far less attractive to me. So in an effort to get the support of people in his own party he is losing the support of people on the Democratic side of the fence. By changing his true colors, perhaps for the first time in his life, he is losing support from people like me who have always admired him for being so true.