For McCain, it’s a political, not a moral decision. He needs to win the middle if he is to win the election. He figures he’s got the conservatives in his pocket (just like Obama has the liberals). They are both fighting over the middle undecideds.
In the end, McCain may choose a pro-life VP. But if he chooses a pro-abortion VP, part of the reason will be he thinks he will gain more votes in the middle than he will lose on the right.
That would be a major miscalculation. If there is one issue that would push conservatives over the edge, this is it.