While others have stated the obvious reasons for why picking a pro-choice vice-presidential nominee would be problematic for John McCain, I think one of the less obvious but still significant reasons why it would be a mistake is that it would reflect on the type of judges McCain would appoint. A huge part of McCain’s outreach efforts to conservatives has involved pledging to appoint strict constructionist judges in the mold of Scalia, Roberts, Alito, and Thomas (for example, see his Wake Forest speech on judicial philosophy). But if, in one of the most important appointments he’ll have to make, McCain disregards values voters and goes with a pro-choice pick, than why would social conservatives consider his reassurances on judges credible? If he ignores their outcry in the midst of an election, why would he care about them once in office, when he no longer needs them?
And I say this as somebody who preferred Giuliani in the primaries. It was pretty clear to me that if either Giuliani, McCain or Romney won the nomination, any one of them would have to pick somebody as their running mate who was a solid conservative on social issues to clear up any doubts (Giuliani obviously would have had the highest bar).