How I see it is a younger VP candidate might persuade moderates concerned about age. The strong conservative will motivate conservatives to actually go out and vote (I know plenty of Republicans who aren't sure whether they're voting or not). This would only help McCain gain votes, and I don't see how it would cause a loss of votes.
McCain is faced with a very tough balancing act. Picking a more conservative VP would help with conservatives, but its going to likely hurt him among Moderates as it will play into the Democratic hands that McCain isn't the independent minded maverick he use to be and is now a party line conservative Republican.
