Maybe, the attacks on McCain leading up to his pick from the religious
right reminded McCain of who he was in 2000, and all the things about himself that he has had to sell in order to get to where he is.
Maybe he realized that if he did win the presidency it wouldn't be his, and the only way to save the nation for four more years was to make sure that the next president wasn't beholden to those forces of corruption.
So he picks the right's most beloved token, someone actually to the right of John McCain on virtually all issues, knowing that she will cause his own campaign to implode, knowing that she will take down the entire neocon-religious right power structure in the republican
party down with her. Knowing that once America gets a good look at the agents of intolerance and hypocrasy that have hijacked his once beloved Republican Party these forces will be exiled from that party forever.
McCain knows he won't ever be president, even if he won, it would still be others who controlled him, so the only way to save America is to destroy his own campaign. The only way to save the Republican Party, the party of Fairness, Freedom and Justice that Lincoln, Roosevelt, and Eisenhower established is to ensure that Barack Obama is the next president of the United States.
Hey it could happen.